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.
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.
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.
The world's biggest known sapphire was formed and shaped by natural processes millions of years ago.