Chalcedony is a form of quartz with a very special structure, known in gemmological circles as cryptocrystalline—meaning the crystal structure is incredibly small and dense, rendering it one of the smoothest gemstones. This allows for the material to be crisply carved, which makes it perfect for the jeweller type for which it is best known—cameos and intaglios.
Chalcedony also comes in a myriad of colours and patterns, and is often better known by these sub-types. One is onyx, a black form of chalcedony, often with white banding. Another is agate, which has fine layers of colour, and was thought by ancient peoples to quench thirst, and protect against fever. One of the most popular is sardonyx, characterized by wider, straight banding, which makes it perfect for use in cameo carving. Bloodstone is deep green colour with red splashes. Carnelian is the name for the reddish orange variety, while jasper is the name of chalcedony of any other colour or pattern.