Silicate of zirconium, also containing thorium and uranium The name may be derived from the Arabic zarkun, meaning "red," or, more probably, from the Persian zargun, golden yellow (hence the English jargon, a variety of zircon).

The presence of radioactive uranium and thorium in the structure sometimes causes a partial disruption of the: crystal lattice of zircon (metamict), with a gradual change in physical properties which, in extreme cases, can be quite pronounced. The varieties used as gems are usually those least affected by this process. They have the highest density and refractive indices and strong birefringence and are referred to, in fact, as “high zircon." Zircons, however, are found with much lower densities and refractive indices, and very weak birefringence. These. specimens in which the crystal structure is badly damaged are pseudomorphs, i.e., they retain their original external shape. Most of them are opaque and cloudy, but some, nearly always green ones, are sufficiently lustrous and transparent to be used as gems. These are known as “low zircon" or metamict zircon. There is a whole series of intermediate forms between the two extremes, transparent specimens almost always being greenish.

Crystal system Tetragonal. -

Appearance Zircon is found in isolated crystals or as twins, in the form of squat prisms with bipyramidal terminations, sometimes cloudy, opaque, but often transparent with considerable luster. They are usually small. They can be light brown or gray, but also brown, yellow, reddish, green, blue, greenish blue, or colorless.

Physical properties The density of high zircon is usually about 4.70 g/cma. Its refractive indices are usually nw 1 .96, ne 2.01 (with marked birefringence, obviously); and it has a hardness of 7.5. Medium to low zircons have a density of between 3.95.and 4.55 g/cm3, refractive indices between flw 1.880, ne 1.890 and mo 1.792, fie 1.796 (with weak birefringence, therefore) and a hardness of about 6-6.5.

Genesis Zircon is generally formed in intrusive magmatic rocks and is also found in the pegmatites derived from them, except in metamorphic schists. Being fairly resistant to the elements, it is also found in the form of small, round pebbles in alluvial deposits.

Occurrence Zircon is found in Norway, Sweden, the Soviet Union (Urals), Australia, Brazil, and the United Staten, but the most famous sources of gem quality zircon are Sri Lanka, Cambodia, Vietnam, and Thailand.