Silicate of calcium and aluminium.
Crystal system Orti-iorhombic.Read More
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. -Read More