geodes.
Gems are produced at very high temperatures, within the layers of carbon, in fact, diamonds are made of high heated carbon, and so are the other multitudes of semi precious stones. The type of stone produced depends on where in the world they are found and how long ago the volcanic action had taken place. Beautiful Blue Sapphires from Ceylon, Rubies from India, Diamonds from Northern Canada and Africa, Pearls from Asia, Opals from Australia, etc., they are all beautiful, and very deadly to reach and extract from the ground.