CHALCEDONY or CALCEDONY, a variety of native silica occurring in concretionary, mammillated or stalactitic forms of waxy lustre and a great variety of colours—though usually white, grey, yellow or brown. It has a compact fibrous structure and a fine splintery fracture. Its relation to quartz has been the subject of a long controversy; one theory supposed it to be a mixture of quartz and opal, whilst the other held that it was a mineral distinct from quartz. Chalcedony fibres are usually, though not invariably, uniaxial and positive, but differ from quartz in that the optic axis is perpendicular to the length. Fur ther, the refractive indices, double refraction, specific gravity and hardness are all slightly less for chalcedony than for quartz. The fine-structure of the two minerals must, however, be very simi lar for their X-ray powder-photographs are identical. This result, which favours the view that chalcedony is a mixture of quartz and opal, is further supported by optical considerations ; for by assigning reasonable values to the density, refractive index and amount of the opal supposed to be present, it is possible to ac count for all the characters of chalcedony.
Chalcedony occurs in veins and amygdales of volcanic rocks together with zeolites and other hydrated silicates. It is deposited from residual magmatic solutions as well as being entirely sec ondary. In this mode of occurrence it is a common mineral in many volcanic fields, as in the basalts of northern Ireland, the Faroe islands, Iceland, the Deccan (India). Chalcedonic pseu domorphs after other minerals often give rise to very interesting specimens. The name "enhydro" is given to hollow nodules of chalcedony containing water and an air bubble which is visible through the semi-transparent wall. In all ages chalcedony has been the stone most used by the gem engraver, and many coloured varieties, described under special headings, are still cut and pol ished as ornamental stones. (See AGATE, BLOODSTONE, CARNELIAN, CHRYSOPRASE, HELIOTROPE, MOCHA STONE, ONYX, SARD and SARDONYX.) (W. A. W.)