Zircon

History and lore

Zircon is among the oldest materials on the planet: tiny grains from the Jack Hills of Western Australia have been dated to about 4.4 billion years, older than any surviving rock. As a gem it has gone by many names. The word zircon is usually derived, through Arabic and German, from the Persian zargun, "gold-coloured". Older European writers called red, orange and brown zircons hyacinth or jacinth, pale and colourless Sri Lankan stones jargoon, and the brilliant colourless stones of southern Sri Lanka "Matara diamonds". These names were never precise: the Greek hyakinthos was originally a blue stone, named after a flower, and medieval "hyacinths" could be sapphires as easily as zircons.

Colourless zircon was long used to imitate diamond, and in the 1920s heat-treated blue zircon from Cambodia and Thailand became fashionable in its own right. Its reputation later suffered from confusion with cubic zirconia, a synthetic zirconium oxide introduced in the 1970s, which is a quite different material.

In the traditions

The King James Bible names "jacinth" as the eleventh foundation of the heavenly Jerusalem (Revelation 21:20), translating the Greek hyakinthos; this may well have meant a blue stone such as sapphire rather than zircon. In the third row of the High Priest's breastplate (Exodus 28:19) the stone rendered "ligure", Hebrew leshem, is of uncertain identity; jacinth, zircon and amber have all been proposed. Medieval lapidaries credited the hyacinth with keeping travellers safe from plague and injury and with bringing sound sleep, and Renaissance correspondence tables gave it to the Sun or to Jupiter, though which stone they meant is often unclear.

In Indian gem practice colourless zircon is widely sold as a substitute (uparatna) for diamond, the Navaratna gem of Venus, and brownish-orange zircon is sometimes offered in place of hessonite, the gem of Rahu; neither is part of the classical Navaratna itself. As a birthstone, zircon was added as an alternative for December in the American revision of 1952, beside turquoise; tanzanite followed in 2002.

Meaning and symbolism

Zircon's modern symbolism draws on its fire and on its age: it is offered as a stone of clarity, endurance and wisdom, and as a reminder of deep time. Crystal-healing books attribute further powers to it, but these are recent ideas, and scientific studies have not found that gemstones heal or change their wearers' fortunes.

The mineral

Zircon crystallises in many igneous and metamorphic rocks and is so resistant to weathering that it survives erosion and collects in gem gravels, notably in Sri Lanka, Myanmar, Cambodia, Thailand, Vietnam, Tanzania and Australia. Its refractive index (about 1.92–2.01 for "high" zircon) and dispersion (about 0.039) come close to diamond's, which gives it strong brilliance and fire. Unlike diamond it is strongly doubly refractive: under a loupe the back facet edges appear doubled, the simplest way to tell the two apart.

Zircon often contains traces of uranium and thorium. Over geological time their radiation damages the crystal lattice, producing "low" or metamict zircon with lower density and duller properties; the same uranium makes zircon the workhorse of uranium–lead dating. Most blue and colourless gem zircons are heat-treated, and a few may revert slightly in strong light. Hardness ranges from about 6.5 to 7.5, and facet edges chip and abrade easily, so zircons should be stored separately and protected from knocks.

Sources and further reading

• Exodus 28; Revelation 21 — King James Bible, 1611
• Natural History, Book 37 — Pliny the Elder, c. 77 CE
• The Curious Lore of Precious Stones — George Frederick Kunz, 1913
• Zircon (Reviews in Mineralogy and Geochemistry 53) — John M. Hanchar and Paul W. O. Hoskin (eds.), 2003
• Gems: Their Sources, Descriptions and Identification (6th ed.) — Michael O'Donoghue (ed.), 2006