Zircon forms in silicate melts with a high concentration of incompatible elements and accepts in its crystal lattice elements with high field strength such as, for example, hafnium which is almost always present in quantities ranging from 1 to 4%. Zircon crystallizes in the tetragonal crystal system . Natural colors generally range from transparent to golden yellow, red, brown, blue and green. Colorless specimens that have the characteristics of gems are used as diamond substitutes . Its polymorphic form is reidite .
There are three varieties: high zircon, the most valuable, which forms prism-shaped crystals, intermediate, which has a crystalline structure partially damaged by the radioactivity of the deposits, and low, whose crystalline structure has been totally damaged by the radioactivity of the deposits.
Zircon, due to uranium and of the thorium often present in the mineral undergoes a more or less strong bombardment. In the most bombarded cases, an almost complete demolition of the crystal lattice is observed, which is why the mineral, from birefringent, becomes singly refractive; in addition, a decrease in hardness, density and refractive index can be observed.
Zircon is one of the oldest gems on Earth. A tiny fragment of it dating back 4.4 billion years has been found in Western Australia, making it the oldest object ever found on Earth.
The mineral is found mainly in pegmatites granite or syenitic, with or without nepheline.
It has a hardness of 7½, close to that of the topaz, and in its colorless variety it has a very high refractive index (1.814-2.024), second only to that of diamond, so its transparency and characteristic sparkle have allowed zircon to become the perfect replacement for the brilliant stone par excellence.
The largest deposits are found in Thailand (Chanthaburi) and in the alluvial deposits of Sri Lanka (in the past zircons were also called Matara diamonds , named after a mining location located on the southern coast of the island of Ceylon). Of particular note are the mines in the Ratanakiri region, whose name in the Khmer language means Gem Mountain , in north-eastern Cambodia, which produces a beautiful variety of deep blue zircon, called precisely Ratanakiri zircon .
Sources: Wikipedia.com