Carnelian is a translucent orange to red chalcedony, microcrystalline quartz, coloured by iron oxide. It grades into sard, a browner variety, and the boundary between the two is conventional rather than sharp. Banded material combining carnelian with white is sardonyx, which matters below.
Much carnelian on the market is heat-treated agate, and this is not a modern deception: heating brown chalcedony to redden it by oxidising the iron is an ancient practice, documented in the Indus Valley, and treated carnelian has been in circulation for four thousand years.
Its distinctive practical property made it the standard material for signet rings in the Greek and Roman world: hot sealing wax does not stick to carnelian, so an intaglio cut into it releases cleanly. That is a straightforward technical reason for a gem's ubiquity, and it accounts for the survival of very large numbers of Roman engraved carnelians. The stone is also relatively easy to engrave for its hardness of around 7, being tough but uniform.
Carnelian is among the oldest worked ornamental stones. It appears in Egyptian amulets from the earliest periods, notably the tjet or knot of Isis, which the Book of the Dead specifies should be made of red stone; in the Royal Cemetery at Ur; and in the Indus cities, where long etched carnelian beads with white patterns produced by an alkali process are a distinctive and widely traded product.
The name is generally derived from Latin cornum, the cornelian cherry, on the colour; a derivation from carnem, flesh, is also offered and is probably a later reworking.