Chemical Composition: Zr(SiO
4)
Mohs' Hardness: 6.5-7.5
Specific Gravity: 3.90 - 4.71
Refractive Index: 1.777-1.987
Double Refraction: +0.059 (none in Green Zircons however)
Dispersion: 0.039
Zircon varies greatly in both color and transparency. Most natural zircons are a brownish mixture of red and yellow. However, clear, yellow, grey, blue, greenish, and darker zircons have been found. The red/yellow zircons are more commonly known as hyacinth. When treated with a heat of 1472-1832°F (800-1000°C) zircon looses it's color and becomes clear or blue. This is done rather often, especially in Asia, and most zircons for sale at a jewelers will be blue or clear, but sometimes the reddish-orange of Hyacinth too. Green zircons are extremely rare, and therefor more expensive. Zircon, while a hard gem, does not always make the best jewelry as it can easily chip and flake. Most zircons for sale are transparent or slightly cloudy.
Hyacinth is often mistaken for another gem, hessonite, and sometimes even colored glass. The easiest way to tell hessonite and hyacinth apart is by their refractiveness. Hessonite is only singly refractive whereas hyacinth is doubly refractive. Glass differs in lustre, specific gravity and hardness, and can be discovered by any tests for one of those properties.
Zircon is found inside igneous and some metamorphic rocks as small crystals or grains. Rarely will more than 1% of the entire mass of a rock consist of zircon. Zircon crystals are tetragonal in shape. Geographically, zircon can be found in Cambodia, Burma, Thailand, Sri Lanka (specifically Myanmar), Australia, The Malagasay republic, Tanzania, Vietnam, France, Maine and South Carolina.
Zircon is usually defined into two different categories, high zircon and low zircon. High zircon is transparent or translucent and has higher marks in both refractive indeces as well as a higher specific gravity. Low zircon has lower numbers in those properties. The large discrepency in values is due to the amount of radioactive elements, such as uranium or thorium, in the stone. However, the amount of radioactivity in the stone will not harm the wearer of a piece of zircon jewelry. Zircon is also in no way associated with Cubic Zirconia, the synthetically made diamond.
Sources:
Precious Stones, by Dr. Max Bauer. Charles E. Tuttle Company: Rutland Vermont and Tokyo, Japan, 1969
Gemstones of the World, by Walter Schumann. Sterling Publishing Co., New York, 1979
http://webmineral.com/data/Zircon.shtml
http://www.mindat.org/show.php?name=Zircon