Peridot

Peridot is gem-quality olivine, a magnesium iron silicate, and it is the counterpart to the sapphire-lapis problem: for much of antiquity, peridot was what the word topaz referred to.

The Greek topazion is generally understood to denote peridot, and the name is traditionally derived from Topazos, an island in the Red Sea: identified with Zabargad, also called St John's Island, off the Egyptian coast, which was the classical source of peridot and is a genuine and important deposit worked for millennia. The stone that modern usage calls topaz, an aluminium fluorosilicate, is a different mineral entirely and was not what the ancient sources meant.

The related term chrysolite, golden stone, is equally unstable: it has meant peridot, chrysoberyl and other yellow-green stones at different times, and it cannot be relied on without context. When Agrippa pairs the star Vega with chrysolite in the Behenian list, the intended mineral is not certainly determinable.

Peridot's colour comes from iron, and it is one of very few gems that occurs in only one colour, it has no colourless or blue variety, because the iron is a constituent of the mineral rather than an impurity. It is comparatively soft at 6.5 to 7 and has a distinctive high birefringence, strong enough that facet edges appear visibly doubled through the stone.

It also has an unusual origin. Olivine is a principal mineral of the earth's mantle, and gem peridot is brought up in basalt or in mantle fragments. It is also found in pallasite meteorites, which contain olivine crystals in an iron-nickel matrix, making peridot one of the few gems that occurs both terrestrially and extraterrestrially.

The name confusion around peridot has a well-known and instructive consequence.

The collection of large green stones in the Three Kings shrine at Cologne Cathedral, long described as emeralds, is peridot. So, on the standard account, was a significant amount of the material called emerald in medieval European treasuries. The mistake ran in the other direction too: Cleopatra's emeralds, from the Egyptian mines of the Eastern Desert, were emerald in at least some cases, but Zabargad peridot circulated in the same markets and the two were not reliably separated by anyone before the modern period. Green was the operative category.

This should govern how any pre-modern gem correspondence is read. When a lapidary assigns virtues to a green stone, those virtues attach to greenness and to a name, not to a mineral species, and the species behind the name may vary between manuscripts, regions and centuries. Reconstructing a stable ancient tradition for a specific mineral is usually not possible, and confident modern statements that a particular species was sacred to a particular people or planet in antiquity are generally doing more than the evidence permits.

Peridot's modern associations (the Sun, Leo, warmth, renewal, the release of resentment), are twentieth-century, drawn from the colour and from the classical epithet gem of the sun. The classical epithet itself is real: the stone was reportedly mined at night, when the material is easier to spot, and worn against night terrors, which Pliny mentions.

Peridot is the August birthstone on the 1912 list; spinel was added alongside it in 2016.