|Chemical composition||ZrSiO4 Zirconium silicate|
|Habit||Prismatic (squared) with pyramidal terminations|
|Optic nature||Uniaxial + to isotropic|
|Refractive index||1.78 - 1.99|
|Birefringence||Up to 0.059|
|Lustre||Vitreous to sub-adamantine|
|Pleochroism||Weak (stronger in heat-treated)|
Zircon is a mineral species that can be broadly defined as a zirconium silicate containing trace amounts of the radioactive minerals hafnium, uranium and thorium. Over time, these radioactive components break down the lattice of the crystal, eventually (over tens of thousands of years) destroying the crystal and leaving it with an amorphous structure and a dark, pithy appearance.
Zircons that are geologically young and unaffected by radioactivity are termed "high" zircons. These stones are transparent golden, yellowish-green and greenish-brown in color with incredibly high dispersion. "High" zircons can be heated to temperatures greater than 982.2 degrees Celsius, when they become colorless or blue.
These highly dispersive colorless stones have long been used as diamond substitutes. That's why the name zircon has the connotation of synthetic or imitation. It was used to imitate a diamond, but the stone is indeed naturally occurring. It should not be confused with the synthetic cubic zirconium (zirconia) or "CZ", which is laboratory created as zirconium oxide and is in no way related to naturally occurring zircon.
Zircons (especially green ones) show a typical uranium spectrum with up to 40 lines. The most diagnostic line will be at 653.5 nm
However some zircons, like metamict type or heat treated white and blue ones, may show only a faint line at 653.5 nm. The red-browns for New South Wales, Australia may not show a spectrum at all.