Carnelian

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.

Carnelian and its relatives sit at the centre of the breastplate identification problem, which is the oldest genuine gem-correspondence question in the Western tradition.

Exodus 28 describes the hoshen, the breastpiece of the high priest, set with twelve stones in four rows, each engraved with the name of a tribe. The Hebrew names are obscure and have been disputed for two thousand years. The first stone, odem, is a red stone and is rendered sardios in the Septuagint, carnelian or sard. That is one of the more secure identifications in the list; most are not.

Two ancient authorities give full identifications and they do not agree with each other. Josephus, in the first century, lists the stones in the Antiquities and also in the Jewish War, and his own two lists differ. Jerome, in the fourth century, gives another set in his letter to Fabiola on the priestly vestments and in the Vulgate. Later medieval lapidaries add more. The idea of correlating the twelve stones with the twelve months and the twelve signs appears in Josephus and is developed by Jerome, and that is the genuine ancient root of birthstone thinking, not the 1912 trade list, which shares almost none of the same stones.

The honest summary is that the breastplate scheme is genuinely ancient, genuinely a stone-and-tribe-and-month correspondence, and genuinely unidentifiable as to which minerals it meant.

Agrippa pairs sardonyx with the fixed star Antares in the Behenian list, which is a separate Renaissance scheme with its own logic, and pairs ruby or garnet with Aldebaran.