Onyx is a microcrystalline variety of Chalcedony which is itself a type of Quartz. It forms in fissures and cavities where silica is precipitated from water over long periods. Once the water flow stops, the material slowly dehydrates leaving behind a mass which gradually mineralises into Chalcedony.

Description: This is an onyx nodule that has been cut in half and polished. The inside consists of a mottled purple and brown matrix with a bright yellow-orange inclusion. The colour is possibly caused by iron.

Location: Atlas Mountains, Morocco.