peridot

Peridot (pronounced, in English gem trade usage, as 'PEHR-ih-doe' with a silent final t, following the French) is gem-quality olivine — a magnesium iron silicate, (Mg,Fe)₂SiO₄ — whose color ranges from yellowish-green to deep olive green to brownish-green, all determined by the proportion of iron in the crystal structure. Unlike most gemstones, which owe their color to trace impurities, peridot is colored by iron that is a fundamental constituent of the mineral itself, which is why it comes only in shades of green: there is no variety of olivine that comes in blue or red or yellow. The English word peridot comes from Old French peridot or peritot, which appears in medieval French lapidary texts, but its pre-French origin is genuinely uncertain. Proposed etymologies include Arabic faridat (gem, precious thing) — which would make peridot a Crusade-era borrowing from Arabic gem trade vocabulary; Greek paederos (a youth, or a stone named for its resemblance to a young face, perhaps from the green tinge of youth); and Medieval Latin pederotus. None of these is fully convincing, and the word's origin remains an acknowledged gap in gem etymology.

Peridot's most celebrated ancient source was an island in the Red Sea known to the ancient Egyptians as Topazios and to Greek and Roman writers as Topazos — the island now called Zabargad (from the Arabic for peridot, zabarjad) or St. John's Island, located about 50 kilometers from the Egyptian coast near Berenice. The confusion encoded in these names is complete: the island was named Topazios because the Greeks called the stone found there topazos (which they confused with yellow topaz); the Arabic name Zabargad names the stone correctly as peridot/zabarjad; and modern gem historians have determined that what Pliny was calling topazius from this island was, mineralogically, peridot. Pliny describes the island as shrouded in fog and accessible only to royal expeditions, the stone guarded zealously against theft. The mines were apparently worked continuously from pharaonic times through the Byzantine period.

The medieval European encounter with peridot came largely through Crusade-era trade contacts with the Islamic world, where zabarjad was prized as a gemstone and recognized for its distinctive color. Many of the large 'emeralds' in medieval European ecclesiastical treasuries — including stones in the Three Kings Shrine in Cologne Cathedral and the Reliquary of the Virgin in Aachen — have been analyzed by modern gemologists and found to be peridot rather than true emerald. The medieval confusion is understandable: peridot's olive green can resemble the greener variety of chrysolite or the paler varieties of emerald, and in the absence of mineralogical testing, color classification was the only tool available. The substitution was not fraud but honest misidentification, reflecting the same color-based classification that makes all ancient gem identifications provisional.

The modern era brought new peridot sources: the San Carlos Apache Reservation in Arizona, which became the world's largest peridot producer in the 20th century; the mines of Pakistan's Kohistan region, which produce fine large crystals; and, most dramatically, the Brenham meteorite site in Kansas and other pallasite meteorites, which contain olivine (peridot) crystals embedded in nickel-iron matrix. Extraterrestrial peridot — olivine crystals that formed in the molten bodies of planetesimals during the early solar system and survived 4.5 billion years to be embedded in a meteorite that landed on Earth — is the oldest gemstone material available to collectors. The green crystal formed in the heart of a planetesimal four and a half billion years ago, traveled through space for billions of years, fell to Earth, and can now be set in a ring. The olive-green color is iron's signature, in a meteorite as in an Egyptian island gem.