– C –
Chalcedony is the cryptocrystalline variety of quartz that is usually pale blue or grey, uniform in tint, but some varieties of which have vari-coloured internal bands or markings. It is porous and so can be (and often is) stained to alter or enhance the colour.
The gemstone varieties include agate, sardonyx, sard, cornelian, chrysoprase, onyx, prase, and many sub-varieties. It is widely imitated in coloured glass, sometimes in several colours blended into each other.