Chalcedony is a form of microcrystalline quartz, composed of extremely fine fibres of silica, which has a waxy lustre and is typically translucent to semi-translucent. Its colours range from milky white, grey and pale blue to brown and red, and the term is used both for the plain stone and as the family name for its many coloured and banded varieties.
Varieties of chalcedony include carnelian, which is orange to red; sard, which is brownish; chrysoprase, coloured green by nickel; bloodstone, green with red spots; and the banded agates and onyx, along with sardonyx, which combines layers of sard and white chalcedony. Because it is hard, takes a fine polish and can be carved in great detail, chalcedony has been used since antiquity for seals, intaglios, cameos, beads, cups and inlay. Roman and Renaissance cameos and intaglios are often carved in banded varieties, which allow contrasting layers to be exploited. In the eighteenth and nineteenth centuries, chalcedony appeared in fob seals, snuff boxes, desk seals, jewellery and Scottish pebble jewellery. Collectors value fine colour, translucency, quality of carving and, in mounted pieces, the quality of the gold or silver settings, while many stones on the market have been artificially dyed.
