From The Gemology Project
Revision as of 08:41, 15 March 2007 by Doos (talk | contribs) (Diagnostics)
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Chemical composition ZrSiO4 Zirconium silicate
Crystal system Tetragonal
Habit Prismatic (squared) with pyramidal terminations
Cleavage Poor
Hardness 7.5
Optic nature Uniaxial + to isotropic
Refractive index 1.78 - 1.99
Birefringence Up to 0.059
Dispersion High, 0.039
Specific gravity 3.90-4.69
Lustre Vitreous to sub-adamantine
Pleochroism Weak (stronger in heat-treated)
Blue Zircon
Photo courtesy of Lembeck Gems

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.


One of the most typical features of zircon (high type) is the high double refraction. White zircon is always heated and that heating restores the crystal lattice to high type, so it will show clear doubling of pavillion facets in faceted stones when viewed through the crown.
As white zircon may mimic a diamond, the "dot test" is a very valuable one. Zircon is the only strong doubly refractive diamond simulant that has a refractive index lower than diamond.


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 from New South Wales, Australia may not show a spectrum at all.

Chelsea Colour Filter

Blue zircons have a green reaction under the CCF.