Peridot — Gem Olivine and the Single-Colour Silicate of August
Peridot — Gem Olivine and the Single-Colour Silicate of August
Iron-coloured magnesium-iron silicate of the olivine series, the August birthstone, and the only major coloured stone with no colour variants of its own
Peridot is gem-quality olivine, a magnesium-iron silicate of the formula (Mg,Fe)2SiO4, distinguished from the rest of the olivine series by its transparency and the saturation of its yellow-green to olive-green colour. The colour comes from iron, which acts as the chromophore at relatively low atomic concentration in a structure dominated by magnesium. Peridot has the unusual distinction of being one of the few major gemstones that occurs in only one colour family — there is no blue peridot or red peridot — and the trade has therefore organised its sense of quality around saturation and tone within the green-to-yellow-green range rather than around colour variants. The species is the modern birthstone for August, a placement reaffirmed by the American Gem Trade Association and Jewelers of America in the standardised birthstone list. Use in jewellery dates to ancient Egypt and the Roman world, where the stone was sourced from the volcanic island of Zabargad in the Red Sea.
Mineralogy and properties
The olivine series runs between forsterite (the magnesium end-member, Mg2SiO4) and fayalite (the iron end-member, Fe2SiO4), with all gem peridot falling in the magnesium-rich half of the series, typically containing 5 to 15 mole-percent fayalite. Crystal system is orthorhombic, hardness is 6.5 to 7 on the Mohs scale, specific gravity ranges from 3.27 to 3.37 with the iron content, and refractive indices fall between approximately 1.65 and 1.69 with a strong birefringence of about 0.036. The high birefringence is responsible for the visible doubling of the back facets when peridot is examined through the table — a diagnostic feature visible to the eye in larger stones.
Cleavage in peridot is poor, an unusually forgiving property for an iron-magnesium silicate, and the stone tolerates faceting and ordinary wear better than the harder but more cleavable topaz. Peridot does, however, have moderate susceptibility to acids and to thermal shock, and ultrasonic and steam cleaning are not recommended.
Sources
The principal modern sources are Pakistan, Myanmar, China, the United States, and the historical Zabargad workings in Egypt. Pakistani peridot from the Sapat valley in the Kohistan region of Khyber Pakhtunkhwa province has, since its discovery in the 1990s, become the standard reference for fine peridot. Pakistani material reaches sizes that the older sources rarely produced — clean stones above 10 carats are routine, and exceptional pieces above 50 and 100 carats appear at intervals — and the Sapat colour is a clean, slightly yellowish green that the Burmese and the older Egyptian production also showed at their best.
Myanmar's Mogok stone tract has produced peridot for centuries alongside its more famous ruby and spinel production. Burmese peridot can match Pakistani material in colour, but reaches significant sizes less commonly. China's principal peridot source is in the Hebei region, and the Chinese material is generally smaller in average size and slightly more yellowish in colour than the Pakistani and Burmese production. The United States produces peridot from the San Carlos Apache Reservation in Arizona — the so-called Peridot Mesa — at scale, with the Apache material accounting for an estimated 80 to 90 percent of world supply by tonnage but generally at smaller sizes (under 5 carats) and at lower per-carat prices than the Pakistani material.
Pallasite peridot is a separate small category. Pallasite meteorites contain olivine crystals embedded in iron-nickel matrix, and the olivine, when of gem quality and recovered from the meteorite, can be faceted into peridot of extraterrestrial origin. The category is novelty rather than commercial mainstream, but is documented in Gems & Gemology and traded by specialist meteorite dealers.
Quality and price
Peridot quality is assessed primarily on colour saturation, with secondary attention to clarity, cut quality, and size. The most desirable colour is a richly saturated grass-green or slightly yellowish grass-green; tones that drift toward olive or brown are less desirable, and very pale or washed-out material is at the bottom of the quality range. Eye-clean clarity is the expectation in commercial-grade material, and the historical Zabargad and modern Pakistani sources both produce essentially eye-clean rough at the better grades. Inclusions when present are commonly the so-called lily pad stress halo around a small black mineral inclusion, a diagnostic feature for peridot.
Per-carat prices for peridot remain moderate by coloured-stone standards. Fine Pakistani material at five carats and above commands the highest prices, with stones above 10 carats of saturated colour reaching premium territory. Apache material at one to three carats supplies the bulk of the commercial jewellery market at modest prices, suitable for August birthstone jewellery and as accent stones in designer work.
In the trade
For the dealer or designer working with peridot, the practical approach is to think about colour saturation first and origin second. The colour difference between fine Pakistani material and routine Apache material is visible to the eye, and the difference is the principal value driver. Origin can be confirmed by laboratory report — Gübelin, SSEF, GIA, AGL all issue origin opinions on peridot — but the trade rarely insists on origin certification at the prices peridot commands, except for unusual large stones.