|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 internal crystal lattice 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 900 degrees Celsius, when they become colorless, blue or golden.
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 in which the crystal lattice has been broken down are named "low type" zircon. These are also named "metamic" as the lattice destruction process through alpha particale decay is termed metamictization.
Zircons that are in a transitional state between high and low type are termed "intermediate" or "medium" type.
One of the most typical features of zircon (high type) is the high double refraction. White zircon is almost always heat treated (from reddish-brown zircon) and this heat treatment also 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 (the double refraction of colorless tourmaline is not as strong as that of high type zircon).
- High and intermediate type zircon:
Colorless, blue, yellow, yellow-green, brownish-green, orangy-brown to orangy-red.
- Low type zircon:
Cloudy brownish-green and yellowish-green (sometimes orange or brown)
Zircons (especially high type green and yellow 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.
Heat treatment of metamict zircon tends to sharpen the faint absorption lines.
Chelsea Colour Filter
Blue zircons have a green reaction under the CCF.
Zircon is routinely heat treated. This heat treatment has two effects on the gem:
- It alters the color to colorless, blue or golden (the most preferred colors for zircon).
- The crystal lattice of metamict zircons is restored to high type zircon.
Heat treatments are carried out in either a oxidizing or a reducing atmosphere (or a sequential combination).
Orangy-brown to reddish-brown zircons ("hyacinth") are heated at around 900° C in an oxidizing atmosphere which turns them yellow to red.
When these brownish zircons are heated at around 1000-1400° C in a reducing environment, it turns them into colorless or blue. Some off-colors are the result of this process aswell.
These off-colored stones are then reheated to around 900° C in oxidizing conditions and this results in colorless, yellow, orange or red colors.
Although colorless, blue and red zircon does exist in nature, they are very rare and there is no known means available to distinguish between them and their heated counterparts.
- Gemstone Enhancement (1984) - Kurt Nassau ISBN 0408014474
- Gems, Their Sources, Descriptions and Identification 4th ed. (1990) - Robert Webster ISBN 0750658568 (6th ed.)