BY ARTHUR THOMAS
The garnets form a complex family of silicates. All its members consist of a combination of divalent and trivalent metallic elements with a typical silicate base.
The ruby red Bohemian pyrope so frequently encountered in Victorian jewellery is a magnesium aluminium silicate with a hue that is enriched by a generous trace of chromium.
From left to right: Pyrope (Magnesium aluminium silicate), Almandine (Iron aluminium silicate), Spessartite (Manganese aluminium silicate)
Magnesium is replaced by iron in the case of the intensely purple almandine garnet and this is responsible for its higher refractive index and specific gravity. The golden-yellow to orange, manganese aluminium silicate, spessartite garnet is even heavier and its refractive index is above the scale of the conventional refractometer.
Rhodolite is a trade name that is applied to slightly-purplish rhododendron-red garnets that are intermediate in composition between pyrope and almandine. There is generally a good market for Tanzanian rhodolite of fine colour in the larger sizes.
Malia is a trade name for complex garnets that may embody molecules of a range of varieties spessartite, almandine, pyrope and quite often grossular. They exhibit a colour range from scarlet to orangy-red.
From left to right: Rhodolite garnet, Mandarin garnet, Malia garnet, Almandine garnet
The discovery of a rare vivid orange spessartite garnet from the Kaokoveld of far north-western Namibia took the Tucson Show by storm. It was christened Mandarin Garnet and the few larger stones that were on offer fetched phenomenal prices.
Purple to violet almandine yields very attractive facetted stones. It is also frequently cut ‘en cabochon’. Fine hornblende needles intersecting at 110deg and 70deg are a common inclusion in almandine garnet and if they are sufficiently dense a correctly oriented cabochon will reveal a four rayed star.
Fellow, Gem-A(GtBr) / Graduate gemologist GIA (USA) / Certified Evaluator (SA)
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