Zircon is a zirconium silicate well-known for its popular gem-quality stones. The name derives from the Arabic word zargun, meaning ‘golden colour’. Zircon is a source of the metal zirconium, which took its name from the mineral and is now used in the production of nuclear reactors.
Zircon is one of the most widely distributed accessory minerals in igneous rocks such as granite, syenite, and nepheline syenite. It occurs in hydrothermal veins in association with minerals such as QUARTZ, FLUORITE, DOLOMITE, PYRITE, SPHALERITE, BARITE, and chalcopyrite. Zircon is also found in metamorphic rocks such as schists and gneisses and becomes concentrated as a detrital mineral in beach and river sands. It is found also in sandstones bearing GOLD.
Gem-quality zircon crystals have an adamantine lustre and are normally found as pebbles in alluvial deposits. In pegmatites, the crystals can reach a considerable size, but most are small, usually prismatic, with bipyramidal terminations. Twinning is common, giving knee-shaped twins. Impurities produce blue, red, brown, green, yellow, and orange varieties. Most zircons contain traces of radioactive uranium or thorium that substitute for zirconium and will eventually break down the crystal structure. These decayed stones, often green, are known as ‘low zircon’.
Sri Lanka has produced gem-quality zircon for more than 2,000 years, and zircon jewellery has been fashioned there and in India for centuries. The gemstone, however, did not become fashionable in western countries until the 1920s. It can be heat-treated to create a variety of colours. Most colourless, blue, and golden stones used in jewellery are heat-treated brown zircons from Thailand, Vietnam, Sri Lanka, and Cambodia. Blue zircon reheated in oxygen will change to a golden yellow colour. Artificially coloured zircons may fade in sunlight over time.
The gem's lustre, hardness, and many colours make it a popular stone for rings. Reddish-brown varieties have been called hyacinth or jacinth. It is most popular as blue, brown, golden, or colourless stones. The colourless and pale stones are usually cut as round and oval brilliants. Reddish-brown hyacinth is also fashioned into step, mixed, baguette and cushion cuts. Transparent zircon is used for gemstones, and colourless zircon resembles DIAMOND and has been sold as such, especially when fashioned as rose cuts in earlier times. Like diamond, it can break up white light into spectral colours. Zircon differs from diamond by its double refraction and, because of its extreme brittleness, by the wear and chipping on the faceted edges of cut stones. Zircon should not be confused with the artificial stone cubic zirconia (cz).
Zircon is widely distributed. Pailin, western Cambodia, is the best source for gem-quality blue zircon. Other top sites for high-quality zircon gems include the Mogok mines in Mandalay, central Burma (Myanmar); Ratnapura, Sri Lanka; and Kanchanaburi, Thailand. Well-formed red crystals come from the Auvergne, France. Brown crystals are found at Arendal, Norway, and near-white rolled pebbles exist in Tanzania. Other sources include Victoria and the beach sands of New South Wales, Australia; Arendal and Langesund, Norway; Kola Peninsula and the gold gravels of the Urals, Russia; Haliburton County and Hastings County, Ontario, Canada; Oberpfalz, Bavaria, and the Eifel Mountains, Rhineland-Palatinate, western Germany; Honshu, Japan; Värmland, Sweden; and at Litchfield, Maine, United States. US deposits include Auburn and Greenwood, Maine; El Paso County, Colorado; Sussex County, New Jersey; and Orange County, New York. Deposits of zircon have also been found on the Moon.