ZIRCON, a mineral composed of zirconium silicate, some times used as a gem-stone. It is believed that the name comes from the Arabic zargun, and is essentially the same as "jargoon," the name given to certain varieties of zircon. The mineral crystal lizes in the tetragonal system, generally in combinations of square prisms and square pyramids (figs. 48 and 49, CRYSTALLOGRAPHY). It is isomorphous with cassiterite and rutile, and like these may form geniculated twins. There is no distinct cleavage, and the mineral breaks with a conchoidal fracture. The hardness is about 7.5. The specific gravity has a very wide range, extending from 4.0 to rather more than 4.7, being thus greater than that of any other gem-stone. Rarely colourless, zircon is usually brown or red, sometimes orange, yellow, green or blue, and occasionally parti-coloured or zoned. Whilst common zircon is opaque, the gem-varieties are transparent.
The effect of heat on zircon is remarkable. Most coloured zir cons, exposed to a high temperature, either change or lose their colour, but this loss is attended by a gain in brilliancy. The "Matura diamonds" of Ceylon are zircons which have been thus artificially decolorized. Certain zircons when heated in a Bunsen flame glow with an orange incandescence, whilst others may emit an orange glow when ground on a copper-wheel fed with diamond dust. Some zircons when heated undergo remarkable changes in
specific gravity and refractive indices, and the suggestion has been made that there are at least three modifications of zirconium sili cate. Zircon is used as a source of zirconia in various prepara tions, for incandescent gas-mantles, etc. It was in this mineral that zirconia was originally discovered by M. H. Klaproth in 1789, whilst it has been found to contain small amounts of hafnium.
When fit for use as a gem-stone it is often known as "noble" or "precious zircon." The red and orange stones are termed hyacinth (q.v.) and jacinth, whilst those of all other colours, as also the colourless transparent zircons, are called jargoons (q.v.). The gem varieties are found in detrital deposits, especially in Ceylon, New South Wales, Queensland and Siam, with sapphire, etc.
Zircon is an accessory constituent of many rocks, especially granite, and it forms an important constituent of the zircon syenite of Norway. Being but little subject to alteration, it is common in secondary deposits, as in auriferous and other sands, occurring usually in small characteristic crystals.