The gem silicates

What holds the family together

These stones do not form a mineralogical family and are not presented as one. Garnet is a cubic silicate group of many species. Peridot is olivine, a magnesium iron silicate from the mantle. Topaz is an aluminium fluorosilicate. Iolite is cordierite. Tanzanite is a variety of zoisite. Kunzite is spodumene, a lithium pyroxene.

What genuinely connects them is optical: all six are transparent enough to be faceted, and all six were valued for what they do with light rather than for the surface a carver could work. That is a real category in the history of the subject, because faceting is comparatively recent. Before the fifteenth century most gems were cut as cabochons or engraved, and the whole aesthetic of brilliance postdates the development of accurate cutting.

Two of them are also unusually informative minerals. Iolite is strongly pleochroic, changing colour with viewing direction. Kunzite fades in sunlight faster than almost any common gem, which is a rare case of an obvious, measurable defect in a gemstone.

How it forms

Garnet forms in metamorphic rocks under heat and pressure and is so reliably diagnostic that geologists use it to read the conditions a rock experienced. Bohemian pyrope from the Czech lands supplied a whole nineteenth-century jewellery industry.

Peridot comes from the mantle, brought up in basalts and, occasionally, in meteorites. The classical source was Zabargad in the Red Sea, worked for at least three and a half thousand years, and Egypt supplied the ancient Mediterranean from there.

Topaz forms in pegmatites and rhyolite cavities. Imperial topaz, the genuinely golden to reddish material, comes from Ouro Preto in Brazil.

Tanzanite comes from a strip of hills a few kilometres long at Merelani near Kilimanjaro, and nowhere else on earth. It was found in 1967 and named by Tiffany.

Kunzite occurs in lithium pegmatites, notably in California, Afghanistan and Brazil, and was described in 1902 by Charles Baskerville and named for George Kunz.

The stones of this family

• Garnet (silicate group, many species, Mohs 7). A whole family, not one stone, named for the pomegranate. Bohemian garnet jewellery, Anglo-Saxon cloisonné and January's birthstone.
• Peridot ((Mg,Fe)₂SiO₄, Mohs 6.5). Olivine of gem quality, mined on Zabargad in the Red Sea since antiquity, and found in meteorites. The only common gem that comes in just one colour.
• Topaz (Al₂SiO₄(F,OH)₂, Mohs 8). Golden imperial topaz from Ouro Preto is the real thing; almost every blue topaz on sale was colourless until it was irradiated. November's birthstone with citrine.
• Iolite (Mg₂Al₄Si₅O₁₈, Mohs 7). Strongly pleochroic, showing violet-blue one way and pale yellow the other. Proposed as the Viking sunstone used to find the sun through cloud.
• Tanzanite (zoisite with vanadium, Mohs 6.5). Found in 1967 at the foot of Kilimanjaro, in one place on earth, and added to December's birthstone list in 2002. Nearly all of it is heated.
• Kunzite (LiAlSi₂O₆, Mohs 6.5). Lithium spodumene, named in 1902 for the gemmologist George Kunz. It fades in sunlight, which is the rare case of a stone that genuinely must be kept in the dark.

Trade names and lookalikes

Blue topaz is the family's headline treatment. Almost all of it began as colourless topaz and was irradiated and then heated to produce blue. The material is safe and regulated, with a cooling period before sale, and it is cheap because colourless topaz is abundant. Natural blue topaz is pale and uncommon.

Tanzanite is heated as a near-universal practice to move brownish zoisite towards violet-blue. Untreated material of good colour is rare.

Mystic topaz and similar rainbow finishes are coated, not natural, and the coating can wear.

Garnet is one of the least treated gems, which is a point in its favour. Synthetic garnets exist, notably YAG and GGG, but they are different compounds used as diamond simulants rather than as garnet substitutes.

Watch for green glass sold as peridot, and for synthetic spinel and cubic zirconia standing in for several members of this group at the low end.

Care that follows from the structure

Hardness varies more than the range suggests. Topaz is 8 but has perfect basal cleavage, so a knock in the wrong direction can split it cleanly, and it should never go in an ultrasonic cleaner. Kunzite has two cleavage directions and is difficult to cut for that reason.

Kunzite also fades. Its pink comes from manganese colour centres that ultraviolet light destroys, and jewellers historically called it an evening stone. Keep it in the dark.

Peridot is only 6.5 and is sensitive to acids and to sudden temperature change; it should not be steamed.

Tanzanite is 6.5 with one cleavage direction and is best set protectively. Garnet is the most robust member and takes daily wear well.

For all of them, warm soapy water and a soft brush is correct, and ultrasonic cleaning should be reserved for garnet alone.