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The olivines near the iron-rich fayalite usually are greenish-yellow, yellow or brown (https://www.mindat.org/min-1458.html). The olivines near the magnesium-rich forsterite usually are green (Fig. 1) and are familiar to many as the gem species peridot (Fig. 2), but they can be colourless, yellow, and even orange (https://www.mindat.org/min-1584.html). The refractive index, as well as other properties like density, varies linearly between fayalite and forsterite, so it is quite easy to determine the composition of a polished specimen by measuring the refractive indicies on a gemmological refractometer. The three illustrated gemstones have upper RIs of 1,687–1,688. This places them at about 12 atomic per cent Fe⁺² along the line between forsterite and fayalite, in a compositional range sometimes called chrysolite (Deer, Howie & Zussman 1996: 6), a term also sometimes used interchangeably with peridot for gem olivine.

So where do gem olivines occur? Although olivine is a common mineral in various different igneous rocks, the gem material is rare. The main historical source was Zagbargad (St John's Island) in the Red Sea. This is thought to be an upthrust portion of Earth’s mantle, and the gem-quality peridot was found in olivine-rich dykes (https://www.mindat.org/loc-6423.html). This leads us to the astonishing fact that magnesium-rich olivine probably is the most common mineral in Earth. It is the major component of the Earth’s mantle, which occupies the greatest volume of the terrestrial sphere. How do we know this? Apart from the olivine-rich dykes on St John’s Island one of the more common occurrences of gem olivine is in volcanic eruptions of basaltic magma originating in the mantle (Fig. 3). These basaltic lavas may contain large chunks of mantle rocks, including nodules consisting almost entirely of bright green olivine rock called dunite (Fig. 4). Where the olivine crystals are large enough they can be recovered as gem rough. The irregular pieces of peridot rough from San Carlos Reservation in Arizona, USA (https://www.mindat.org/loc-52628.html) are extracted from just such nodules in basaltic lava (Fig. 5). Kimberlite, one of the host rocks of diamond, also originates in the upper mantle. It too contains plenty of olivine, hence the name of the matrix of the rock, peridotite (https://geology.com/rocks/peridotite.shtml).

Figure 3. A dunite nodule consisting almost entirely of olivine, in black basaltic lava from Lanzerote, Canary Islands. The specimen is 65 × 55 × 35 mm.


Figure 4. Basalt containing several nodules of dunite or peridotite, with varying amounts of forsterite olivine, from the San Carlos Reservation in Arizona, USA. This specimen is 150 × 110 × 65 mm.


Figure 5. Peridot gem rough from San Carlos Reservation in Arizona, USA. This parcel weighs 75,8 g.

Another modern source of peridot is from serpentinised shear zones in dunite. This is the geological setting of gem olivine crystals from Sapat Gali, Kagan Valley, Pakistan (https://www.mindat.org/loc-2536.html). Here the olivine crystals grew into their characteristic orthorhombic morphology, with only slightly rounded edges and lightly etched faces (Fig. 6). They make very collectable specimens, often too good for use as faceting rough. Other magnesium-rich olivines occur in thermally metamorphosed dolomites and dolomitic limestones. The forsterite crystals embedded in calcite from Kolonne in Sri Lanka in the example illustrated in Figure 7 are only black on the outside; within they are yellow-green (https://www.mindat.org/loc-275464.html). The most exotic source of olivine for gemstones is from rare pallasite meteorites. These are fragments of one or more disrupted asteroids, consisting of iron and large crystals of yellow olivine. Some very rare gemstones have been cut from this material (https://geology.com/gemstones/gems-from-space/).

Figure 6. The peridot crystals from Sapat Gali in Pakistan are among the best olivine specimens available. This cluster of gem-quality crystals is 40 × 32 × 20 mm.


Figure 7. An example of well-crystallised forsterite olivine in calcite, from Kolonne, Sri Lanka. The whole specimen is 55 × 40 × 20 mm.

Closer to home, olivine forms visible crystals in the so-called olivine melilitite in various intrusive bodies in the Western and Northern Cape. This rather enigmatic rock-type also originates in the Earth’s mantle. In thin slabs the yellow olivine crystals form little rectangular windows. Fifteen years ago the club visited two of these – the Goedemoed pipe at Van Loveren Private Cellar near Robertson and the dyke-like body associated with the Salpeterkop intrusion at Sutherland (Miller 2005). Discarded pieces of this dull-looking greenish rock can still be found abandoned in the garden bed around the club house.

A common occurrence of olivine, but not gem-quality peridot, is in metal smelting slags. Iron-rich fayalite often forms a major component of the waste from metal smelting (Fig. 8). Here the crystals are minute, but their compositions are fayalitic and under the microscope they have characteristic yellow internal reflections. In slags that have cooled rapidly they form beautiful, arrow-shaped skeletal crystals, once again revealing their orthorhombic morphology (Fig. 9).

Figure 8. This is a micrograph of a polished section of an iron smelting slag from the Iron Age site of Bosutswe in Botswana, viewed in reflected plane-polarised light. The yellowish-grey, skeletal, fayalite olivine crystals form spear-like laths, growing in a dark glass with finer fayalite crystals, and bright dendritic crystals of magnetite. The field of view is 0,8 mm, so the largest olivine crystals are only about 1 mm long. This slag must have cooled rapidly from a fully molten state.

Figure 9. This is a micrograph of a thin section of a copper smelting slag from an indigenous smelting site near Rohoboth in Namibia, viewed in crossed-polarised light. The skeletal fayalite crystals grew in a dark, copper-rich glass. The bright, intense interference colours are typical of olivine but are not the true colours of the crystals, which are colourless in thin section. The field of view is 5 mm.

Figure 10. A gold necklace with five peridot gems in my Southern Cross cut, made by Cape Town jeweller Michael Cope for a private client of his (photograph by Michael Cope)

Of course, peridot also can be found in the stock of mineral dealers, gem cutters and in jewellery (Fig. 10), where you don’t have to dig to find it, but just swap electrons or sheets of coloured paper to add it to your private mineral collection (Fig. 11). 

  
Parallel growth peridot, Burma, 25mm high               St John’s Island peridot, 30mm high
Collection Jo Wicht

References:

Deer, W. A., Howie, R. A. & Zussman, J. 1966. An introduction to the rock forming minerals. Longman: London.

Miller, D. 2005. The Sutherland and Robertson olivine melilitites. South African Lapidary Magazine 37(3): 21–25.