Olivine — The Magnesium-Iron Silicate of the Upper Mantle
Olivine — The Magnesium-Iron Silicate of the Upper Mantle
The mineral group whose gem variety, peridot, has been treasured since antiquity
Olivine is a magnesium-iron silicate mineral group with the general formula (Mg,Fe)2SiO4, forming a continuous solid-solution series between the magnesium endmember forsterite (Mg2SiO4) and the iron endmember fayalite (Fe2SiO4). It is one of the most abundant minerals in the silicate Earth, dominating the composition of the upper mantle and a major rock-forming mineral in basaltic and ultramafic igneous rocks. Gem-quality olivine, with composition close to the magnesium-rich forsterite end of the series, is known by the trade name peridot — one of the few gem species whose ancient name persists in modern usage. Olivine has been mined for gemstones for at least three and a half thousand years, and the species occupies a distinctive position both in geology and in the gem trade.
Mineralogy and properties
Olivine crystallises in the orthorhombic system, with hardness 6.5 to 7 on the Mohs scale, specific gravity around 3.27 to 3.37 (rising with iron content along the forsterite-fayalite series), and refractive indices of approximately 1.64 to 1.69. The fracture is conchoidal, with poor cleavage; toughness is fair to good for a stone of this hardness. The colour ranges from yellow-green through the bottle-green of fine peridot to the brownish green of more iron-rich material. Pure forsterite is colourless to pale yellow; pure fayalite is yellowish brown to brown; gem peridot is in the yellowish-green to green range, with the green colour produced principally by Fe2+ in the crystal structure.
Strong birefringence — visible doubling of facet edges when a peridot is viewed face-up through the table — is a diagnostic feature that distinguishes olivine from species that might be confused with it visually. The doubling effect is one of the simplest field-level identifications for peridot.
Geological occurrence
Olivine is the dominant mineral of the upper mantle, comprising approximately 50 to 60 per cent of mantle peridotite by volume. Mantle xenoliths brought to the surface in basaltic eruptions and kimberlite pipes contain abundant olivine, sometimes of gem quality where the fragment has not been altered by transport processes. Most economic gem-peridot deposits are in basaltic rocks where the olivine has been concentrated by partial melting of the mantle and subsequent eruption, although some deposits — including the historically important Zabargad Island production — are from peridotite massifs that have been exhumed and weathered.
Olivine also occurs in ultramafic and mafic igneous rocks of the lower crust and in some metamorphic rocks. The mineral is unstable at the surface in the presence of water and weathers readily to serpentine, talc, and other secondary minerals; this is why fresh, unaltered olivine of gem quality is relatively concentrated rather than universally distributed.
Major deposits
Historic peridot production came from Zabargad Island (also called St John's Island) in the Red Sea off the Egyptian coast, where ancient and mediaeval mining produced the famous Zabargad peridots. The island was effectively the world's only significant source for peridot from antiquity through the early twentieth century. From the late nineteenth century, additional sources developed in the United States — the San Carlos Apache Indian Reservation in Arizona — and in Myanmar (Mogok), Pakistan (the high Himalayan deposits at Sapat and Suppat), and China.
The Pakistan deposits at Sapat in the Kohistan region of the upper Indus Valley have produced some of the finest peridot of the modern period, including stones of significant size with strong saturated colour. The Myanmar Mogok production is smaller but has yielded fine stones. The San Carlos production is the volume leader, supplying the bulk of commercial peridot in calibrated sizes.
Position in the trade
Peridot is the August birthstone and a familiar fixture in commercial jewellery, with prices generally moderate compared to the major coloured-stone species. Fine large peridots — particularly Pakistani material above five carats with strong saturated colour and good clarity — command meaningful prices but remain affordable compared to fine emerald, ruby, or sapphire of equivalent size. The species is often described as undervalued by collectors who appreciate its colour but find it more accessible than the other major coloured stones.
Care
Peridot's hardness of 6.5 to 7 makes it adequate but not exceptional for ring use; protected settings and careful handling extend its working life. Ultrasonic cleaning is generally acceptable for clean stones, but stones with stress or significant inclusions may suffer; mild soap and warm water is always appropriate. Sudden thermal shock should be avoided, and the stone should not be subjected to acids that can attack the silicate structure.