tag:theconversation.com,2011:/au/topics/my-favourite-gem-56779/articles
My favourite gem – The Conversation
2018-08-23T20:03:43Z
tag:theconversation.com,2011:article/101115
2018-08-23T20:03:43Z
2018-08-23T20:03:43Z
More than just a sparkling gem: what you didn’t know about diamonds
<p><em><strong><a href="https://theconversation.com/au/topics/my-favourite-gem-56779">My favourite gem</a></strong> is an occasional series in which we ask a scientist to share the fascinating geological and social features of a beautiful rock.</em></p>
<hr>
<p>They’re made of carbon – but there’s something almost supernatural about diamonds. </p>
<p>Just the word diamond invokes luxury, desirability and toughness. Yet when we think of the element carbon we are more likely to think of charcoal; soft, black, opaque, earthy, light-weight. </p>
<figure class="align-right ">
<img alt="" src="https://images.theconversation.com/files/232028/original/file-20180815-2897-nhgph1.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=237&fit=clip" srcset="https://images.theconversation.com/files/232028/original/file-20180815-2897-nhgph1.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=600&h=398&fit=crop&dpr=1 600w, https://images.theconversation.com/files/232028/original/file-20180815-2897-nhgph1.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=600&h=398&fit=crop&dpr=2 1200w, https://images.theconversation.com/files/232028/original/file-20180815-2897-nhgph1.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=600&h=398&fit=crop&dpr=3 1800w, https://images.theconversation.com/files/232028/original/file-20180815-2897-nhgph1.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=754&h=501&fit=crop&dpr=1 754w, https://images.theconversation.com/files/232028/original/file-20180815-2897-nhgph1.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=754&h=501&fit=crop&dpr=2 1508w, https://images.theconversation.com/files/232028/original/file-20180815-2897-nhgph1.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=754&h=501&fit=crop&dpr=3 2262w" sizes="(min-width: 1466px) 754px, (max-width: 599px) 100vw, (min-width: 600px) 600px, 237px">
<figcaption>
<span class="caption">Charcoal is also made from carbon – but it hasn’t been subjected to enormous pressures like diamond.</span>
<span class="attribution"><a class="source" href="https://www.shutterstock.com/image-photo/artists-black-charcoal-smudge-55675423?src=aphPUvFdEiA3tEstZCayJQ-1-12">from www.shutterstock.com</a></span>
</figcaption>
</figure>
<p>It’s fascinating to see how the crystalline arrangement of carbon atoms transform when subjected to pressures greater than about 40 kilobars (the equivalent of 40,000 Earth atmospheres). These conditions are experienced at depths in the earth from about 120km down. </p>
<p>And some diamonds come from way, way deeper – more than 650km (about the distance from Canberra to Melbourne) into the Earth. Tiny imperfections in such diamonds give us clues about what’s happening in the Earth’s hidden geological layers. </p>
<hr>
<p>
<em>
<strong>
Read more:
<a href="https://theconversation.com/what-is-a-gem-and-why-painite-from-myanmar-can-fetch-us-60-000-per-carat-97453">What is a gem? And why painite from Myanmar can fetch US$60,000 per carat</a>
</strong>
</em>
</p>
<hr>
<p>In contrast with carbon in its low pressure form as charcoal or graphite, carbon atoms in diamond are fixed together in a strong, three dimensional network. This leads to unique physical properties: diamond is a clear, extremely hard, often colourless mineral with a very high density. </p>
<p>Diamonds sparkle and have internal fire because of their very high refractive index. This means light is “caught” inside the crystal and re-reflected off the internal surfaces. Faces and facets made by gem cutters accentuate this property. </p>
<h2>Violently erupted to the surface</h2>
<p>Although diamonds have been prized as valuable gems for a long time, until the early 1700s virtually all traded diamonds came from <a href="https://www.bloomsbury.com/uk/koh-i-noor-9781408888841/">river gravels</a> (known as “alluvial deposits”) in India. </p>
<p>Then in the early eighteenth century diamonds were <a href="https://www.sciencedirect.com/science/article/pii/0264370795000174">discovered in Brazil</a>, and from 1866 onwards were mined in South Africa. It was in this country that diamond’s major, violently erupted, volcanic source rock known as “kimberlite” was identified <a href="https://www.springer.com/gp/book/9781461358220">for the first time</a>. </p>
<p>This recognition fundamentally changed the diamond exploration and mining industry, and quickly led to vastly increased production and to the high demand from the modern jewellery industry. </p>
<p>Supply of diamonds to the market has long been tightly controlled by a small number of major producers – examples include De Beers (South Africa-Botswana), Al Rosa (Russia), Rio Tinto (Argyle Mine Australia and Canadian mines) and Lucara Diamond Corporation (Karowe Mine, Botswana).</p>
<figure class="align-center ">
<img alt="" src="https://images.theconversation.com/files/232226/original/file-20180816-2891-zhrwea.png?ixlib=rb-1.1.0&q=45&auto=format&w=754&fit=clip" srcset="https://images.theconversation.com/files/232226/original/file-20180816-2891-zhrwea.png?ixlib=rb-1.1.0&q=45&auto=format&w=600&h=462&fit=crop&dpr=1 600w, https://images.theconversation.com/files/232226/original/file-20180816-2891-zhrwea.png?ixlib=rb-1.1.0&q=30&auto=format&w=600&h=462&fit=crop&dpr=2 1200w, https://images.theconversation.com/files/232226/original/file-20180816-2891-zhrwea.png?ixlib=rb-1.1.0&q=15&auto=format&w=600&h=462&fit=crop&dpr=3 1800w, https://images.theconversation.com/files/232226/original/file-20180816-2891-zhrwea.png?ixlib=rb-1.1.0&q=45&auto=format&w=754&h=581&fit=crop&dpr=1 754w, https://images.theconversation.com/files/232226/original/file-20180816-2891-zhrwea.png?ixlib=rb-1.1.0&q=30&auto=format&w=754&h=581&fit=crop&dpr=2 1508w, https://images.theconversation.com/files/232226/original/file-20180816-2891-zhrwea.png?ixlib=rb-1.1.0&q=15&auto=format&w=754&h=581&fit=crop&dpr=3 2262w" sizes="(min-width: 1466px) 754px, (max-width: 599px) 100vw, (min-width: 600px) 600px, 237px">
<figcaption>
<span class="caption">The Diavik kimberlite pipe in northern Canada.</span>
<span class="attribution"><span class="source">John Foden</span>, <span class="license">Author provided</span></span>
</figcaption>
</figure>
<h2>The value of diamonds</h2>
<p>Unlike other mined commodities such as copper, gold, oil or coal, diamond has no spot market. Its value is variable and highly subjective, assessed using the “4C” system: colour, clarity, cut and carat (5 carat = 1 gram). </p>
<p>Per carat, uncut diamond values typically vary from around $US10 to $US3000. Very large (sometimes very historical) gem-quality diamonds however may command price orders of magnitude beyond this. </p>
<p>The intensely blue 45.5 carat <a href="https://historicalnovelsociety.org/reviews/hope-adventures-of-a-diamond/">Hope Diamond</a> started its traded history in India in the early 1600s, and is valued at more than US$200 million. Other recent <a href="https://www.ritani.com/blog/diamonds/most-expensive-diamonds/">high-priced diamond sales</a> include the Pink Star (59.6 carats, $US71 million) and the Oppenheimer Blue (14.6 carats, $US57.5 million). </p>
<p>The largest diamond recently sold is the uncut Botswanan 1,109 carat diamond, the “Lesedi La Rona”. This sold for <a href="https://www.cnbc.com/2017/09/26/large-diamond-the-lesedi-la-rona-sells-for-53-million.html">$US53 million</a>. </p>
<h2>Clues about diamond origins</h2>
<p>Many diamonds contain inclusions of other minerals, which are captured samples from the deep Earth rocks in which the diamond grew. These provide important information for geologists. </p>
<p>For example, inclusions of the minerals olivine, pyroxene and garnet tell us their host diamonds grew at depths between about 120 and 300km, in a layer of the Earth known as the <a href="http://rsta.royalsocietypublishing.org/content/360/1800/2383">sub-continental lithospheric mantle</a>. </p>
<p>This layer is part of the Earth’s continental tectonic plates, and lies below the oldest regions of Earth’s continental crust known as “<a href="https://agupubs.onlinelibrary.wiley.com/doi/abs/10.1029/RG013i003p00001">cratons</a>”. Cratons are up to four billion years old - examples include the Australian Pilbara, the South African Kaapvaal, the Canadian Slave and the Russian Siberian craton. </p>
<h2>Blue diamonds deep, deep down</h2>
<p>Although the sub-continental lithospheric mantle is the most common source of diamonds, some come from much deeper layers in the Earth. </p>
<p>These are called <a href="http://eprints.gla.ac.uk/11307/">sub-lithospheric diamonds</a>, and identified by mineral inclusions consistent with being exposed to much higher pressures found at depths of more than 650km. </p>
<p>A <a href="https://www.nature.com/articles/s41586-018-0334-5">recent study</a> looked at a type of rare blue diamond like the Hope Diamond. The researchers consistently detected very high pressure mineral inclusions indicating their diamond hosts grew at depths of at least 660km. These diamonds are blue because of the presence of trace amounts of the element boron. </p>
<figure class="align-center zoomable">
<a href="https://images.theconversation.com/files/232224/original/file-20180816-2906-c0rjwn.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=1000&fit=clip"><img alt="" src="https://images.theconversation.com/files/232224/original/file-20180816-2906-c0rjwn.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=754&fit=clip" srcset="https://images.theconversation.com/files/232224/original/file-20180816-2906-c0rjwn.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=600&h=568&fit=crop&dpr=1 600w, https://images.theconversation.com/files/232224/original/file-20180816-2906-c0rjwn.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=600&h=568&fit=crop&dpr=2 1200w, https://images.theconversation.com/files/232224/original/file-20180816-2906-c0rjwn.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=600&h=568&fit=crop&dpr=3 1800w, https://images.theconversation.com/files/232224/original/file-20180816-2906-c0rjwn.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=754&h=713&fit=crop&dpr=1 754w, https://images.theconversation.com/files/232224/original/file-20180816-2906-c0rjwn.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=754&h=713&fit=crop&dpr=2 1508w, https://images.theconversation.com/files/232224/original/file-20180816-2906-c0rjwn.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=754&h=713&fit=crop&dpr=3 2262w" sizes="(min-width: 1466px) 754px, (max-width: 599px) 100vw, (min-width: 600px) 600px, 237px"></a>
<figcaption>
<span class="caption">The Hope Diamond started its traded history in India in the early 1600s.</span>
<span class="attribution"><a class="source" href="https://mineralsciences.si.edu/collections/hope/gallery.htm">Smithsonian National Museum of National History</a></span>
</figcaption>
</figure>
<p>The question of how boron ended up at great depths in the Earth’s mantle is a fascinating one. Boron is an element that on Earth is highly concentrated in the upper continental crust (less than 20km deep) and in ocean water. Its concentration in deeper mantle rocks is typically extremely low.</p>
<p>Boron then must have been re-introduced to the deep layers where the diamonds grew. </p>
<p>This would likely have happened through a process called deep subduction, where the boundary of an oceanic tectonic plate (about 100km thick) fails, and the plate then collapses into the deep earth’s mantle. This moves boron and other materials from the shallow layers of the Earth down into depths of over 700 km.</p>
<p>Kimberlite eruptions then bring the diamonds up towards the surface. </p>
<figure class="align-center ">
<img alt="" src="https://images.theconversation.com/files/233194/original/file-20180823-149463-ad5ani.jpeg?ixlib=rb-1.1.0&q=45&auto=format&w=754&fit=clip" srcset="https://images.theconversation.com/files/233194/original/file-20180823-149463-ad5ani.jpeg?ixlib=rb-1.1.0&q=45&auto=format&w=600&h=506&fit=crop&dpr=1 600w, https://images.theconversation.com/files/233194/original/file-20180823-149463-ad5ani.jpeg?ixlib=rb-1.1.0&q=30&auto=format&w=600&h=506&fit=crop&dpr=2 1200w, https://images.theconversation.com/files/233194/original/file-20180823-149463-ad5ani.jpeg?ixlib=rb-1.1.0&q=15&auto=format&w=600&h=506&fit=crop&dpr=3 1800w, https://images.theconversation.com/files/233194/original/file-20180823-149463-ad5ani.jpeg?ixlib=rb-1.1.0&q=45&auto=format&w=754&h=636&fit=crop&dpr=1 754w, https://images.theconversation.com/files/233194/original/file-20180823-149463-ad5ani.jpeg?ixlib=rb-1.1.0&q=30&auto=format&w=754&h=636&fit=crop&dpr=2 1508w, https://images.theconversation.com/files/233194/original/file-20180823-149463-ad5ani.jpeg?ixlib=rb-1.1.0&q=15&auto=format&w=754&h=636&fit=crop&dpr=3 2262w" sizes="(min-width: 1466px) 754px, (max-width: 599px) 100vw, (min-width: 600px) 600px, 237px">
<figcaption>
<span class="caption">Subduction of oceanic lithosphere with boron (B) captured from the oceans and delivered by the subducting oceanic slab to lower mantle depths in excess of 660km. Here the boron is supplied to the growing ultra-high pressure sub-lithospheric diamonds.</span>
<span class="attribution"><span class="source">John Foden</span>, <span class="license">Author provided</span></span>
</figcaption>
</figure>
<h2>A window into deep Earth</h2>
<p>In addition to the boron example above, evidence from other diamond mine sites also supports the idea that Earth elements move from relatively shallow to deeper into the Earth through the process of subduction. </p>
<p>This has been detected by tracking different forms of carbon in diamonds from the <a href="https://www.nature.com/articles/nature25972">South African Cullinan mine</a>, and in my own research on mineral inclusions in <a href="https://pubs.geoscienceworld.org/gsa/geology/article/37/1/43/193506/deep-mantle-diamonds-from-south-australia-a-record">South Australian diamonds</a>.</p>
<figure class="align-center ">
<img alt="" src="https://images.theconversation.com/files/232228/original/file-20180816-2906-1esf9t9.png?ixlib=rb-1.1.0&q=45&auto=format&w=754&fit=clip" srcset="https://images.theconversation.com/files/232228/original/file-20180816-2906-1esf9t9.png?ixlib=rb-1.1.0&q=45&auto=format&w=600&h=446&fit=crop&dpr=1 600w, https://images.theconversation.com/files/232228/original/file-20180816-2906-1esf9t9.png?ixlib=rb-1.1.0&q=30&auto=format&w=600&h=446&fit=crop&dpr=2 1200w, https://images.theconversation.com/files/232228/original/file-20180816-2906-1esf9t9.png?ixlib=rb-1.1.0&q=15&auto=format&w=600&h=446&fit=crop&dpr=3 1800w, https://images.theconversation.com/files/232228/original/file-20180816-2906-1esf9t9.png?ixlib=rb-1.1.0&q=45&auto=format&w=754&h=560&fit=crop&dpr=1 754w, https://images.theconversation.com/files/232228/original/file-20180816-2906-1esf9t9.png?ixlib=rb-1.1.0&q=30&auto=format&w=754&h=560&fit=crop&dpr=2 1508w, https://images.theconversation.com/files/232228/original/file-20180816-2906-1esf9t9.png?ixlib=rb-1.1.0&q=15&auto=format&w=754&h=560&fit=crop&dpr=3 2262w" sizes="(min-width: 1466px) 754px, (max-width: 599px) 100vw, (min-width: 600px) 600px, 237px">
<figcaption>
<span class="caption">A South Australian diamond with visible inclusions.</span>
<span class="attribution"><span class="source">John Foden</span>, <span class="license">Author provided</span></span>
</figcaption>
</figure>
<p>Deep parts of the Earth still have a physical connection with layers closer to the surface.</p>
<p>So yes diamonds are valuable due to being beautiful, hardy and relatively rare – but they also provide a fantastic window into the structure and the history of our Earth.</p><img src="https://counter.theconversation.com/content/101115/count.gif" alt="The Conversation" width="1" height="1" />
<p class="fine-print"><em><span>John Foden receives funding from Australian Research Council. </span></em></p>
Some diamonds come from depths of more than 650km. Tiny imperfections in these gems give us clues about what’s happening in Earth’s hidden geological layers.
John Foden, Professor, University of Adelaide
Licensed as Creative Commons – attribution, no derivatives.
tag:theconversation.com,2011:article/97084
2018-08-09T20:07:47Z
2018-08-09T20:07:47Z
A disappointing earring, and the world’s hottest rock: zirconia
<figure><img src="https://images.theconversation.com/files/225645/original/file-20180702-116152-d01eb5.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=496&fit=clip" /><figcaption><span class="caption">Diamond or zirconia? Apart from the price, it can be hard to tell these two gems apart. </span> <span class="attribution"><a class="source" href="https://www.shutterstock.com/image-photo/close-beautiful-woman-wearing-vinyage-shiny-513160801">from www.shutterstock.com </a></span></figcaption></figure><p><em><strong><a href="https://theconversation.com/au/topics/my-favourite-gem-56779">My favourite gem</a></strong> is an occasional series in which we ask a scientist to share the fascinating geological and social features of a beautiful rock.</em></p>
<hr>
<p>Just last week, my partner handed me an earring that she had found in a park near her home more than seven years ago, and for which she’d had no luck finding the owner.</p>
<p>The earring had a gold setting that gripped a very large, transparent, beautifully cut gemstone. Given that I am a geologist, she asked me to check it out for her to find out more about the stone. After all, we thought, if it was a diamond, then it could fund a holiday to somewhere lovely, given its large size. Hawaii, perhaps? </p>
<hr>
<p>
<em>
<strong>
Read more:
<a href="https://theconversation.com/why-we-value-diamond-rings-and-other-valentines-day-gifts-89056">Why we value diamond rings and other Valentine's Day gifts</a>
</strong>
</em>
</p>
<hr>
<p>Upon analysing the earring under an electron microscope, it was immediately apparent that the earring wasn’t a real diamond. It turned out to be cubic zirconia. </p>
<p>Funnily enough, zirconia is a topic very close to my heart and my professional expertise. In 2017 I wrote a <a href="https://www.sciencedirect.com/science/article/pii/S0012821X1730451X">research paper</a> on the “world’s hottest rock”, and zirconia was part of that story. </p>
<figure class="align-center ">
<img alt="" src="https://images.theconversation.com/files/221701/original/file-20180605-175445-18iw252.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=754&fit=clip" srcset="https://images.theconversation.com/files/221701/original/file-20180605-175445-18iw252.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=600&h=782&fit=crop&dpr=1 600w, https://images.theconversation.com/files/221701/original/file-20180605-175445-18iw252.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=600&h=782&fit=crop&dpr=2 1200w, https://images.theconversation.com/files/221701/original/file-20180605-175445-18iw252.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=600&h=782&fit=crop&dpr=3 1800w, https://images.theconversation.com/files/221701/original/file-20180605-175445-18iw252.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=754&h=983&fit=crop&dpr=1 754w, https://images.theconversation.com/files/221701/original/file-20180605-175445-18iw252.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=754&h=983&fit=crop&dpr=2 1508w, https://images.theconversation.com/files/221701/original/file-20180605-175445-18iw252.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=754&h=983&fit=crop&dpr=3 2262w" sizes="(min-width: 1466px) 754px, (max-width: 599px) 100vw, (min-width: 600px) 600px, 237px">
<figcaption>
<span class="caption">A false colour image of a grain of zircon (yellow) that has formed a rim of zirconia (multicolours) at 2,370°C when the surrounding rock melted during an ancient meteorite impact. This is the hottest known temperature to be achieved by any rock naturally at Earth’s surface.</span>
<span class="attribution"><span class="source">Nick Timms</span>, <span class="license">Author provided</span></span>
</figcaption>
</figure>
<h2>Crystal lattice gems</h2>
<p>Zirconia is a mineral with a crystal structure made from the elements zirconium (Zr) and oxygen (O), with the chemical formula ZrO₂. It looks quite a lot like diamond, but is only worth a fraction of the value because it is manufactured and not a rare natural gem. </p>
<p>Cubic zirconia belongs to a family of zirconia minerals, each having a different configuration of atoms that give rise to different crystal lattice structures, called polymorphs. Similarly, graphite and diamond are polymorphs of carbon – each made entirely of carbon but with different structures. </p>
<p>Like diamond, cubic zirconia is stable at very high temperatures and pressures.</p>
<p>But unlike its sparkly friend diamond, cubic zirconia isn’t stable at Earth’s surface. In fact, it has never been found in natural rocks, which is why it hasn’t been given a proper mineral name by geologists. </p>
<hr>
<p>
<em>
<strong>
Read more:
<a href="https://theconversation.com/what-is-a-gem-and-why-painite-from-myanmar-can-fetch-us-60-000-per-carat-97453">What is a gem? And why painite from Myanmar can fetch US$60,000 per carat</a>
</strong>
</em>
</p>
<hr>
<p>When zirconia is manufactured for making jewellery, the manufacturers use additives to stabilise it. Impurities are incorporated into the mineral and act like atomic scaffolding, holding the structure together so that it resists transforming into one of the other zirconia polymorphs. </p>
<p>If it is pure, though, it readily morphs into one of its siblings – first to a form known as tetragonal zirconia, then to baddeleyite – a polymorph of zirconia that is stable at room temperature. Every time the atoms shift to transform into a different polymorph, they leave evidence behind within the crystal. </p>
<h2>Earth’s hottest rocks</h2>
<p>So what has this got to do with the hottest rocks on Earth’s crust? </p>
<p>I am interested in finding out how Earth responds when asteroids hit it. I have studied a few of the 192 meteorite impact craters that have been discovered on Earth, including the one that was ultimately responsible for ending the existence of the dinosaurs. </p>
<p>Australia has its fair share of craters – Wolfe Creek is particularly well known, made famous by a chilling horror movie. </p>
<hr>
<p>
<em>
<strong>
Read more:
<a href="https://theconversation.com/target-earth-how-asteroids-made-an-impact-on-australia-92836">Target Earth: how asteroids made an impact on Australia</a>
</strong>
</em>
</p>
<hr>
<p>Canada’s ancient landscape has also accumulated many scars of bombardment. One particular ancient impact structure – Mistastin Lake in Labrador – contains the solidified remnants of a lava flow near its centre that forms huge cliffs called Discovery Hill. </p>
<figure class="align-center ">
<img alt="" src="https://images.theconversation.com/files/229920/original/file-20180731-176708-1al832i.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=754&fit=clip" srcset="https://images.theconversation.com/files/229920/original/file-20180731-176708-1al832i.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=600&h=575&fit=crop&dpr=1 600w, https://images.theconversation.com/files/229920/original/file-20180731-176708-1al832i.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=600&h=575&fit=crop&dpr=2 1200w, https://images.theconversation.com/files/229920/original/file-20180731-176708-1al832i.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=600&h=575&fit=crop&dpr=3 1800w, https://images.theconversation.com/files/229920/original/file-20180731-176708-1al832i.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=754&h=723&fit=crop&dpr=1 754w, https://images.theconversation.com/files/229920/original/file-20180731-176708-1al832i.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=754&h=723&fit=crop&dpr=2 1508w, https://images.theconversation.com/files/229920/original/file-20180731-176708-1al832i.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=754&h=723&fit=crop&dpr=3 2262w" sizes="(min-width: 1466px) 754px, (max-width: 599px) 100vw, (min-width: 600px) 600px, 237px">
<figcaption>
<span class="caption">Mistastin Lake in Canada sits in a meteorite impact crater.</span>
<span class="attribution"><span class="source">Google Maps</span>, <span class="license">Author provided</span></span>
</figcaption>
</figure>
<p>The energy released by the meteorite impact was enough to melt and even vaporise the rocks at ground zero, which then cooled quickly (quenched) to form the glassy black rocks of the cliffs. </p>
<figure class="align-center zoomable">
<a href="https://images.theconversation.com/files/229922/original/file-20180731-176708-1ieoise.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=1000&fit=clip"><img alt="" src="https://images.theconversation.com/files/229922/original/file-20180731-176708-1ieoise.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=754&fit=clip" srcset="https://images.theconversation.com/files/229922/original/file-20180731-176708-1ieoise.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=600&h=138&fit=crop&dpr=1 600w, https://images.theconversation.com/files/229922/original/file-20180731-176708-1ieoise.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=600&h=138&fit=crop&dpr=2 1200w, https://images.theconversation.com/files/229922/original/file-20180731-176708-1ieoise.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=600&h=138&fit=crop&dpr=3 1800w, https://images.theconversation.com/files/229922/original/file-20180731-176708-1ieoise.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=754&h=173&fit=crop&dpr=1 754w, https://images.theconversation.com/files/229922/original/file-20180731-176708-1ieoise.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=754&h=173&fit=crop&dpr=2 1508w, https://images.theconversation.com/files/229922/original/file-20180731-176708-1ieoise.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=754&h=173&fit=crop&dpr=3 2262w" sizes="(min-width: 1466px) 754px, (max-width: 599px) 100vw, (min-width: 600px) 600px, 237px"></a>
<figcaption>
<span class="caption">The impact structure at Discovery Hill, Mistastin Lake – where the world’s hottest rock was found. CLICK TO VIEW.</span>
<span class="attribution"><span class="source">Mike Zanetti, Western University, Canada.</span>, <span class="license">Author provided</span></span>
</figcaption>
</figure>
<p>The rocks had the texture of syrup when I examined them under the microscope, and I could easily see that only a few fragments of the most physically robust minerals had escaped being completed melted. </p>
<p>Frozen in this quenched rock soup were a few little remnant grains of the mineral zircon (with the chemical formula ZrSiO₄). The zircon must have been present in the target rocks before the catastrophic event, and had taken a “hot bath” when they became immersed in the lava. </p>
<p>However, zircon is a tough cookie and doesn’t melt when it is heated. Instead, and at high enough temperatures, it decomposes to form tiny crystals of zirconia and liquid silica. The specs of zircon in this particular rock had begun to decompose, reacting within the hot magma and had become encrusted with a beautiful rind of zirconia (the baddeleyite form).</p>
<figure class="align-center ">
<img alt="" src="https://images.theconversation.com/files/229926/original/file-20180731-176711-1qocy3i.png?ixlib=rb-1.1.0&q=45&auto=format&w=754&fit=clip" srcset="https://images.theconversation.com/files/229926/original/file-20180731-176711-1qocy3i.png?ixlib=rb-1.1.0&q=45&auto=format&w=600&h=465&fit=crop&dpr=1 600w, https://images.theconversation.com/files/229926/original/file-20180731-176711-1qocy3i.png?ixlib=rb-1.1.0&q=30&auto=format&w=600&h=465&fit=crop&dpr=2 1200w, https://images.theconversation.com/files/229926/original/file-20180731-176711-1qocy3i.png?ixlib=rb-1.1.0&q=15&auto=format&w=600&h=465&fit=crop&dpr=3 1800w, https://images.theconversation.com/files/229926/original/file-20180731-176711-1qocy3i.png?ixlib=rb-1.1.0&q=45&auto=format&w=754&h=585&fit=crop&dpr=1 754w, https://images.theconversation.com/files/229926/original/file-20180731-176711-1qocy3i.png?ixlib=rb-1.1.0&q=30&auto=format&w=754&h=585&fit=crop&dpr=2 1508w, https://images.theconversation.com/files/229926/original/file-20180731-176711-1qocy3i.png?ixlib=rb-1.1.0&q=15&auto=format&w=754&h=585&fit=crop&dpr=3 2262w" sizes="(min-width: 1466px) 754px, (max-width: 599px) 100vw, (min-width: 600px) 600px, 237px">
<figcaption>
<span class="caption">Rock collected from the top of Discovery Hill, on the southweastern edge of Mistastin Lake (shown in the photographs above).</span>
<span class="attribution"><span class="source">Mike Zanetti, Western University, Canada</span>, <span class="license">Author provided</span></span>
</figcaption>
</figure>
<p>My research team could read the paper trail in the baddeleyite, detecting the former presence of cubic zirconia from which the baddeleyite had transformed. This meant that this rock had once been at a blistering 2,370°C during the impact event around 38 million years ago. This is the <a href="https://www.sciencedirect.com/science/article/pii/S0012821X1730451X">hottest temperature ever recorded</a> for a rock anywhere on Earth’s surface. </p>
<p>This finding made us wonder what might Earth have gone through early in its history, when it was being bombarded frequently by similar-sized or even bigger impacts, and its surface was subject to these extreme temperatures on a regular basis.</p>
<p>There’s an amazing sense of excitement and awe from making discoveries from studying tiny mineral fragments in rocks that spark such deep thoughts about events and timescales almost beyond comprehension. Luckily, this can offset the mild feeling of disappointment after analysing a found earring. </p>
<p>Another time, Hawaii, another time! </p>
<hr>
<p>
<em>
<strong>
Read more:
<a href="https://theconversation.com/life-death-and-politics-in-hawaii-125-years-of-colonial-rule-90273">Life, death and politics in Hawaii: 125 years of colonial rule</a>
</strong>
</em>
</p>
<hr>
<img src="https://counter.theconversation.com/content/97084/count.gif" alt="The Conversation" width="1" height="1" />
<p class="fine-print"><em><span>Nick Timms does not work for, consult, own shares in or receive funding from any company or organisation that would benefit from this article, and has disclosed no relevant affiliations beyond their academic appointment.</span></em></p>
Zirconia is a mineral with a crystal structure made from the elements zirconium and oxygen. It looks pretty like diamond, but is only worth a fraction of the value.
Nick Timms, Senior Lecturer, Curtin University
Licensed as Creative Commons – attribution, no derivatives.
tag:theconversation.com,2011:article/98474
2018-08-08T20:09:10Z
2018-08-08T20:09:10Z
Sapphire secrets: they aren’t all blue, and mining them requires luck plus labour
<figure><img src="https://images.theconversation.com/files/224385/original/file-20180622-26546-1abhc5f.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=496&fit=clip" /><figcaption><span class="caption">Sapphire beads in a Jaipur workshop. </span> <span class="attribution"><span class="source">Lynda Lawson </span>, <span class="license">Author provided</span></span></figcaption></figure><p><em><strong><a href="https://theconversation.com/au/topics/my-favourite-gem-56779">My favourite gem</a></strong> is an occasional series where we ask a scientist to share the fascinating geological and social features of a beautiful rock.</em></p>
<hr>
<p>I first remember seeing sapphires as a teenager in a jeweller’s shop in Silver Street in pre-Khmer Rouge Phnom Penh – the deep saturated blues of the gems from Palin on the border with Thailand were captivating. The sapphires my father bought that day are still in the family long after any trace of Silver Street has disappeared. </p>
<p>It was not until recently when I met a female sapphire miner in Madagascar that I began to appreciate the hard labour involved in the mining of these stones across Africa and Asia.</p>
<hr>
<p>
<em>
<strong>
Read more:
<a href="https://theconversation.com/what-is-a-gem-and-why-painite-from-myanmar-can-fetch-us-60-000-per-carat-97453">What is a gem? And why painite from Myanmar can fetch US$60,000 per carat</a>
</strong>
</em>
</p>
<hr>
<h2>Inside sapphires</h2>
<p>Sapphires are crystals of the mineral corundum, made up mostly of atoms of aluminium and oxygen in a 2 to 3 ratio (Al₂0₃). </p>
<p>The chemical bonds of aluminium and oxygen are particularly tight, making the sapphire one of the hardest minerals known – 9 out of 10 on a measure of hardness used for minerals known as <a href="https://geology.com/minerals/mohs-hardness-scale.shtml">Moh’s scale</a>. Sapphires are second to diamonds in hardness. </p>
<p>This quality makes the stone valuable for jewellery, as it keeps its shine and does not scratch. </p>
<p>The aluminium and oxygen do not reflect light, making sapphires transparent, another characteristic that is highly prized in a gemstone.</p>
<p>The tight chemical bonds of corundum also make the stones very dense and relatively heavy. This means they are not swept away during weathering, but accumulate in alluvial deposits – that is, left behind by the flow of water. As a result, sapphires can be found by miners tunnelling down to ancient riverbeds or in bends of existing river beds. </p>
<figure>
<iframe width="440" height="260" src="https://www.youtube.com/embed/Oq-rqbkZo0s?wmode=transparent&start=0" frameborder="0" allowfullscreen=""></iframe>
<figcaption><span class="caption">Understanding coloured gemstones.</span></figcaption>
</figure>
<h2>Different shades of blue</h2>
<p>The colour of corundum comes not from the aluminium or the oxygen, but is due to <a href="https://www.youtube.com/watch?v=j1Y2E237ZbM">impurities and inclusions in the crystalline structure</a>. </p>
<p>The blue of sapphires is from substitution of both iron and titanium in place of some of the aluminium. The shade of blue will depend on the ratio of ferrous iron and ferric iron, with higher levels of ferrous iron producing the inky blues characteristic of Australian sapphires. </p>
<p>Corundum that is red is known as ruby – and results from the presence of chromium substituting for aluminium in the crystal structure. </p>
<p>Yellow sapphires are often caused by the presence of iron without the involvement of titanium. </p>
<p>Star sapphires – rare sapphires which reflect a star in certain light conditions – occur when fine needle-like inclusions of minerals such as rutile are aligned parallel to the six sides of the hexagonal crystal shape of a sapphire. When cut correctly, light reflects causing a six rayed star to appear. </p>
<h2>The black star of Queensland</h2>
<p><a href="https://www.sustainablegemstones.org/videos/">Sapphires and rubies</a> were created many millions of years ago through one of two geological processes: metamorphic (involving rock transformation) or magmatic (associated with volcanoes).</p>
<p>Metamorphic stones grew during ancient mountain building events such as continental collision – where heat and pressure caused the crystallisation of gem quality corundum. </p>
<p>Metamorphic sapphire and ruby deposits are among the finest in the world. Traditionally they have been found in Sri Lanka, Mogok and Kashmir and more recently in Mozambique, Tanzania and especially Madagascar. </p>
<p>Magmatic sapphires and rubies are not associated with a mountain building event but were carried to the surface in volcanic events from deep in the earth. Magmatic sapphires and rubies occur in eastern Australia, Southeast Asia, Africa, and North America. </p>
<p>Queensland has produced some of the world’s most famous sapphires: the <a href="https://jewelry-blog.internetstones.com/the-black-star-of-queensland-famous-black-sapphire-gemstone/">Black Star of Queensland</a> at 1,156 carats (uncut), is the largest black sapphire ever found.</p>
<figure class="align-center ">
<img alt="" src="https://images.theconversation.com/files/229917/original/file-20180731-176698-x1htq7.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=754&fit=clip" srcset="https://images.theconversation.com/files/229917/original/file-20180731-176698-x1htq7.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=600&h=400&fit=crop&dpr=1 600w, https://images.theconversation.com/files/229917/original/file-20180731-176698-x1htq7.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=600&h=400&fit=crop&dpr=2 1200w, https://images.theconversation.com/files/229917/original/file-20180731-176698-x1htq7.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=600&h=400&fit=crop&dpr=3 1800w, https://images.theconversation.com/files/229917/original/file-20180731-176698-x1htq7.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=754&h=503&fit=crop&dpr=1 754w, https://images.theconversation.com/files/229917/original/file-20180731-176698-x1htq7.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=754&h=503&fit=crop&dpr=2 1508w, https://images.theconversation.com/files/229917/original/file-20180731-176698-x1htq7.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=754&h=503&fit=crop&dpr=3 2262w" sizes="(min-width: 1466px) 754px, (max-width: 599px) 100vw, (min-width: 600px) 600px, 237px">
<figcaption>
<span class="caption">Now about the size of a large egg, the Black Star of Queensland sapphire was once used as a doorstop.</span>
<span class="attribution"><a class="source" href="https://commons.wikimedia.org/wiki/File:Black_Star_of_Queensland_Star_Sapphire.jpg">Wikimedia Commons</a></span>
</figcaption>
</figure>
<p>It was discovered on the Anakie sapphire fields in Central Queensland in 1938 by 12 year old Roy Spencer. His father used it as a door stop, until it was bought by an American collector, and cut and polished into a magnificent oval cabochon which displays a six point star. Its current value could be as much as <a href="https://www.thenaturalsapphirecompany.com/blog/the-black-star-of-queensland">US$50 million</a>. </p>
<p>Today there is a strong demand for fine quality sapphires from East Asian and European markets. Especially prized are the velvety blues of Kashmir sapphires, and the pink orange hues of the padparadscha sapphire (named after the lotus flower). </p>
<hr>
<p>
<em>
<strong>
Read more:
<a href="https://theconversation.com/how-gold-rushes-helped-make-the-modern-world-91746">How gold rushes helped make the modern world</a>
</strong>
</em>
</p>
<hr>
<h2>The social cost of sapphires</h2>
<p>Jewellers and consumers in the West are increasingly interested in the origin of their stones and to know that <a href="https://www.vogue.com.au/fashion/accessories/how-the-jewellery-industry-is-cleaning-up-its-act/news-story/424bff47fc08bb20d7395559becbb0c6">human rights have been respected</a> in their mining and cutting. </p>
<p>Industry bodies such as <a href="https://www.responsiblejewellery.com/">The Responsible Jewellery Council</a> are seeking to create standards for coloured gemstones, which take into account the social and environmental impact of mining and the sustainability of the industry. </p>
<figure class="align-center ">
<img alt="" src="https://images.theconversation.com/files/227558/original/file-20180713-27036-1mfy790.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=754&fit=clip" srcset="https://images.theconversation.com/files/227558/original/file-20180713-27036-1mfy790.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=600&h=448&fit=crop&dpr=1 600w, https://images.theconversation.com/files/227558/original/file-20180713-27036-1mfy790.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=600&h=448&fit=crop&dpr=2 1200w, https://images.theconversation.com/files/227558/original/file-20180713-27036-1mfy790.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=600&h=448&fit=crop&dpr=3 1800w, https://images.theconversation.com/files/227558/original/file-20180713-27036-1mfy790.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=754&h=563&fit=crop&dpr=1 754w, https://images.theconversation.com/files/227558/original/file-20180713-27036-1mfy790.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=754&h=563&fit=crop&dpr=2 1508w, https://images.theconversation.com/files/227558/original/file-20180713-27036-1mfy790.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=754&h=563&fit=crop&dpr=3 2262w" sizes="(min-width: 1466px) 754px, (max-width: 599px) 100vw, (min-width: 600px) 600px, 237px">
<figcaption>
<span class="caption">A family group sieving for sapphires in South West Madagascar.</span>
<span class="attribution"><span class="source">Lynda Lawson</span>, <span class="license">Author provided</span></span>
</figcaption>
</figure>
<p><a href="https://www.sustainablegemstones.org/">The GemHub project</a> – a collaboration between the Universities of Queensland, Delaware and Lausanne, and funded by Tiffany and Co Foundation – seeks to improve knowledge about coloured gemstones and sustainability issues linked to the mining, processing and trading of these precious resources. </p>
<p>My work with this project has taken me to the world’s gem stone hub of Jaipur to investigate the health and safety of gem cutters, particularly the risk of silicosis, an occupational lung disease caused by the <a href="https://theconversation.com/explainer-what-is-silicosis-and-why-is-this-old-lung-disease-making-a-comeback-80465">inhalation of silica</a>. </p>
<p>Today this has largely been controlled by a process of wet cutting and clean up, but the problem still remains in smaller shops among agate and quartz cutters, especially in cottage industries in Gujarat.</p>
<hr>
<p>
<em>
<strong>
Read more:
<a href="https://theconversation.com/explainer-what-is-silicosis-and-why-is-this-old-lung-disease-making-a-comeback-80465">Explainer: what is silicosis and why is this old lung disease making a comeback?</a>
</strong>
</em>
</p>
<hr>
<p>I have also worked with remote Madagascan communities of sapphire mining women on a basic field gemmology training course written and delivered by young Australian geologist <a href="https://www.lawsongems.com/">Charles Lawson</a>. </p>
<p>The women miners’ and traders’ negotiating skills are greatly enhanced by knowing their stones and being able to use simple tools such as tweezers and a loupe (a small magnifying glass). This has been financially empowering to the women, and they are now starting to organise their own trading and polishing shop. </p>
<figure class="align-center zoomable">
<a href="https://images.theconversation.com/files/227559/original/file-20180713-27039-yr4nb2.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=1000&fit=clip"><img alt="" src="https://images.theconversation.com/files/227559/original/file-20180713-27039-yr4nb2.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=754&fit=clip" srcset="https://images.theconversation.com/files/227559/original/file-20180713-27039-yr4nb2.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=600&h=450&fit=crop&dpr=1 600w, https://images.theconversation.com/files/227559/original/file-20180713-27039-yr4nb2.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=600&h=450&fit=crop&dpr=2 1200w, https://images.theconversation.com/files/227559/original/file-20180713-27039-yr4nb2.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=600&h=450&fit=crop&dpr=3 1800w, https://images.theconversation.com/files/227559/original/file-20180713-27039-yr4nb2.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=754&h=566&fit=crop&dpr=1 754w, https://images.theconversation.com/files/227559/original/file-20180713-27039-yr4nb2.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=754&h=566&fit=crop&dpr=2 1508w, https://images.theconversation.com/files/227559/original/file-20180713-27039-yr4nb2.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=754&h=566&fit=crop&dpr=3 2262w" sizes="(min-width: 1466px) 754px, (max-width: 599px) 100vw, (min-width: 600px) 600px, 237px"></a>
<figcaption>
<span class="caption">Malagasy sapphire miner learning to use loupe and tweezers to identify her stone.</span>
<span class="attribution"><span class="source">Lynda Lawson</span>, <span class="license">Author provided</span></span>
</figcaption>
</figure>
<p>However, there is still some way to go before women and men working at the very first stage of the sapphire value chain receive equitable compensation for their work in mining and trading these rare and wonderful stones. </p>
<p>The price for small rough stones can be under a dollar a carat. When cut and polished, the best of these stones can fetch up to AU$100 a carat and fine, larger stones many times that.</p><img src="https://counter.theconversation.com/content/98474/count.gif" alt="The Conversation" width="1" height="1" />
<p class="fine-print"><em><span>Lynda Lawson receives funding from
Tiffany and Co Foundation and
Deutsche Gesellschaft für Internationale Zusammenarbeit GmbH (GIZ) , a German development agency</span></em></p>
Sapphires and rubies are both crystals of the mineral corundum - but with different impurities to create blue and red hues. Australian sapphires are renowned for being inky blue.
Lynda Lawson, PhD candidate , The University of Queensland
Licensed as Creative Commons – attribution, no derivatives.
tag:theconversation.com,2011:article/97453
2018-08-01T04:34:37Z
2018-08-01T04:34:37Z
What is a gem? And why painite from Myanmar can fetch US$60,000 per carat
<figure><img src="https://images.theconversation.com/files/230128/original/file-20180731-136646-k0u6lv.gif?ixlib=rb-1.1.0&q=45&auto=format&w=496&fit=clip" /><figcaption><span class="caption">The first three Natural History Museum painites – including one in its natural state with rubies that had been sitting in their collection for years. It had initially been misidentified as the much less valuable tourmaline.</span> <span class="attribution"><a class="source" href="http://www.nhm.ac.uk/">© The Trustees of the Natural History Museum, London</a>, <span class="license">Author provided</span></span></figcaption></figure><p><em><strong><a href="https://theconversation.com/au/topics/my-favourite-gem-56779">My favourite gem</a></strong> is an occasional series where we ask a scientist to share the fascinating geological and social features of a beautiful rock.</em></p>
<hr>
<p>Humans have adorned themselves and their belongings with attractive stones since prehistoric times. We’ve used fossil materials such as jet and amber, colourful rocks such as lapis lazuli, and water-clear single crystals of minerals such as amethyst and golden citrine. </p>
<p>The “precious stones” diamond, ruby, sapphire and emerald are distinguished from the remaining “semiprecious stones” largely on the basis of perceived rarity in classical times.</p>
<p>But what makes a stone a gem? It boils down to a few key qualities – beauty and durability. And rarity makes a gem even more special, as is the case for my favourite: painite. </p>
<hr>
<p>
<em>
<strong>
Read more:
<a href="https://theconversation.com/why-we-value-diamond-rings-and-other-valentines-day-gifts-89056">Why we value diamond rings and other Valentine's Day gifts</a>
</strong>
</em>
</p>
<hr>
<h2>Tough beauty</h2>
<p>Any stone may become a gem if it has beauty (in the eyes of enough beholders) and is durable enough to retain that beauty through everyday wear. </p>
<p>Durability usually implies that the stone is hard enough to resist abrasion from airborne sand and dust. Also, that it does not easily fracture or “cleave” on flat planes of weakness (determined by its atomic arrangement). </p>
<p>Diamond, the hardest known material, certainly satisfies the abrasion criterion. A diamond crystal does have four orientations of cleavage plane on which it can be split easily. But for diamonds, this apparent liability can be turned into an asset. </p>
<p>The cleavage is used as a short cut in the early stages of shaping, cutting and polishing this extraordinarily hard material, which is otherwise a slow and painstaking business. </p>
<figure>
<iframe width="440" height="260" src="https://www.youtube.com/embed/OhFRhfOedXA?wmode=transparent&start=0" frameborder="0" allowfullscreen=""></iframe>
<figcaption><span class="caption">How to cleave a diamond.</span></figcaption>
</figure>
<h2>The four Cs</h2>
<p>Demand drives the value of gemstones as commercial items, and this in turn is a function of fashion and name recognition. </p>
<p>However, the main valuation criteria for gems such as diamonds are often summarised as “the four Cs”: <a href="https://www.gemsociety.org/article/gem-pricing-guide-sample/">carats, colour, clarity and cut</a>. </p>
<p>One carat (0.2 g) is the traditional unit of weight for a gemstone – but larger stones are disproportionately rare, and worth more per weight than smaller ones. </p>
<p>While pure diamonds are colourless, and the same is true of many other gem minerals, striking and rare colours almost always increase their value. </p>
<p>Small amounts of impurities or defects of the crystal structure are needed to produce the prized pink diamonds for which the Argyle mine of northwestern Australia is famous. </p>
<p>Impurities also turn the common mineral corundum into its red form (ruby) and other coloured varieties familiar as sapphires. Such colours are appreciated best through the depths of a transparent, intact single crystal, with the passage of light unimpeded by fractures, inclusions or rough surfaces. Hence the value of clarity. </p>
<h2>The rare gem painite</h2>
<p>Although diamonds are still the popular epitome of preciousness, <a href="https://io9.gizmodo.com/5902212/ten-gemstones-that-are-rarer-than-diamond?IR=T">they are far from the rarest minerals to have been used as gems</a>. </p>
<p>As a mineralogist, my favourite amongst these ultra-rare stones comes from the gem gravels of the Mogok region in Myanmar. There, sapphires, rubies, spinels and other gemstones accumulate in river beds after washing down from the surrounding forested hills. These have been mined since ancient times. </p>
<p>In 1957, two deep red stones from a batch donated to the Natural History Museum in London – as shown in the lead image for this story – turned out to be completely new to science. A tiny slice from one crystal was used for research, and the <a href="http://minerals.gps.caltech.edu/files/Visible/painite/Index.html">new mineral was named “painite”</a> after the original donor, the gem dealer Arthur Pain.</p>
<hr>
<p>
<em>
<strong>
Read more:
<a href="https://theconversation.com/de-beers-to-sell-synthetic-diamonds-heres-how-theyre-made-97558">De Beers to sell synthetic diamonds: here’s how they’re made</a>
</strong>
</em>
</p>
<hr>
<p>A third painite was identified in 1979, but it was not until 2001 that a fourth was found in Myanmar. Efforts to find more intensified, working uphill along creeks and locating progressively less water-worn material. </p>
<p>By 2005, a source outcrop for painite was finally discovered, nearly half a century after the original identification. Several thousand stones have now been recovered, but the small number of cut gems remains the preserve of specialist collectors. </p>
<p>Painite’s extreme rarity is due to it containing the chemical elements zirconium and boron, which do not normally associate with each other in nature and don’t occur together in any other mineral. Ironically, some painite crystals are partly altered to a crust of small pink crystals of the more common ruby. </p>
<p>The increase in supply means that you can now get small crystals of painite pretty easily online for tens of dollars, and poor-quality cut stones for about A$100. However, the tiny proportion of gem-quality stones still fetch <a href="http://www.jewelsdujour.com/2012/08/ten-rarest-precious-stones/">US$60,000 per carat</a>. </p>
<h2>Opal, the odd one out</h2>
<p>The national gemstone of Australia, precious opal, is an anomaly. It is soft enough to scratch easily, <a href="https://www.ajsgem.com/articles/how-care-your-opal-gemstones.html">prone to cracking</a>, most attractive when nearly opaque, and does not occur as crystals.</p>
<figure class="align-center ">
<img alt="" src="https://images.theconversation.com/files/227121/original/file-20180711-27036-16tz1k7.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=754&fit=clip" srcset="https://images.theconversation.com/files/227121/original/file-20180711-27036-16tz1k7.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=600&h=401&fit=crop&dpr=1 600w, https://images.theconversation.com/files/227121/original/file-20180711-27036-16tz1k7.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=600&h=401&fit=crop&dpr=2 1200w, https://images.theconversation.com/files/227121/original/file-20180711-27036-16tz1k7.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=600&h=401&fit=crop&dpr=3 1800w, https://images.theconversation.com/files/227121/original/file-20180711-27036-16tz1k7.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=754&h=504&fit=crop&dpr=1 754w, https://images.theconversation.com/files/227121/original/file-20180711-27036-16tz1k7.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=754&h=504&fit=crop&dpr=2 1508w, https://images.theconversation.com/files/227121/original/file-20180711-27036-16tz1k7.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=754&h=504&fit=crop&dpr=3 2262w" sizes="(min-width: 1466px) 754px, (max-width: 599px) 100vw, (min-width: 600px) 600px, 237px">
<figcaption>
<span class="caption">A rare example of boulder opal.</span>
<span class="attribution"><a class="source" href="https://www.shutterstock.com/image-photo/rare-boulder-opal-coober-pedy-australia-1037378152?src=gfrGT4QniUAL0dRvj2X-gw-1-7">from www.shutterstock.com</a></span>
</figcaption>
</figure>
<p>Opal is made from microscopic spheres of non-crystalline silica in a very regular array. This natural “<a href="http://www.uvm.edu/%7Edahammon/Structural_Colors/Structural_Colors/Opals_And_Photonic_Crystals.html">photonic crystal</a>” diffracts light to produce the play of rainbow colours whose beauty overcomes all other considerations.</p>
<figure class="align-center ">
<img alt="" src="https://images.theconversation.com/files/227119/original/file-20180711-27018-ss54mj.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=754&fit=clip" srcset="https://images.theconversation.com/files/227119/original/file-20180711-27018-ss54mj.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=600&h=400&fit=crop&dpr=1 600w, https://images.theconversation.com/files/227119/original/file-20180711-27018-ss54mj.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=600&h=400&fit=crop&dpr=2 1200w, https://images.theconversation.com/files/227119/original/file-20180711-27018-ss54mj.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=600&h=400&fit=crop&dpr=3 1800w, https://images.theconversation.com/files/227119/original/file-20180711-27018-ss54mj.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=754&h=503&fit=crop&dpr=1 754w, https://images.theconversation.com/files/227119/original/file-20180711-27018-ss54mj.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=754&h=503&fit=crop&dpr=2 1508w, https://images.theconversation.com/files/227119/original/file-20180711-27018-ss54mj.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=754&h=503&fit=crop&dpr=3 2262w" sizes="(min-width: 1466px) 754px, (max-width: 599px) 100vw, (min-width: 600px) 600px, 237px">
<figcaption>
<span class="caption">Opal mining creates health hazards in Coober Pedy, South Australia.</span>
<span class="attribution"><a class="source" href="https://www.shutterstock.com/image-photo/danger-sign-coober-pedy-south-australia-129058475?src=XEzqJMV1SOiRSajmg5elZw-1-14">from www.shutterstock.com</a></span>
</figcaption>
</figure>
<hr>
<p>
<em>
<strong>
Read more:
<a href="https://theconversation.com/from-mine-to-wine-creative-uses-for-old-holes-in-the-ground-3245">From mine to wine: creative uses for old holes in the ground</a>
</strong>
</em>
</p>
<hr>
<h2>Where science comes in</h2>
<p>The polishing and carving of semiprecious stones originated in prehistory. </p>
<p>But the cutting of diamonds in particular has become a sophisticated craft well grounded in the science of optics. One rough stone may ultimately produce several finished stones of different sizes, the pattern of cuts being chosen to minimise waste. </p>
<p><a href="http://www.folds.net/diamond_design/index.html#fig_37">Diamond cutters angle the facets on each stone precisely</a>, so as to maximise the internal reflection of light and the dispersion of white light into rainbow sparkles. They also aim to produce an overall shape which appeals best to the market. </p>
<p>Similar considerations have led to standard cuts being developed for other gems, to show them at their best.</p><img src="https://counter.theconversation.com/content/97453/count.gif" alt="The Conversation" width="1" height="1" />
<p class="fine-print"><em><span>Andrew Christy does not work for, consult, own shares in or receive funding from any company or organisation that would benefit from this article, and has disclosed no relevant affiliations beyond their academic appointment.</span></em></p>
What makes a stone a gem? It boils down to a few key qualities: beauty and durability. But opal, the national gemstone of Australia, is an anomaly - it’s soft.
Andrew Christy, Senior Curator (Mineralogy) at the Queensland Museum and Lecturer, The University of Queensland
Licensed as Creative Commons – attribution, no derivatives.