Treatment of Zircon Gems

Zircon gems occur naturally in a variety of colors - including a light blue, a greenish blue, colorless to deep yellow, red and the most commonly found brown coloration. At one time colorless zircon was widely used as a diamond imitation but it's now been replaced by synthetics with far superior hardness and optical properties. As a result, colorless zircons are now rarely made through heat treatment. Zircon crystals occur in what is known as a high and low crystalline state, but nearly all of the gem quality varieties of zircon are forms of the high crystal state.

Blue zircons, which are very popular and well known gems, are produced almost exclusivly through heat treatment. This process was discovered roughly about a century ago, and has been popular since that time. To turn them blue, the stones must be heated to about 1000°C in a strongly reducing atmosphere for a period of several hours.  Normally this is accomplished by including coal or charcoal both inside the heating crucible and in the surrounding furnace area.  Experience shows that in general, the more strongly reducing the atmosphere the darker the final color will be. This blue color is stable and will not change during average jewelry wear conditions. Not all Zircon crystals will take on the blue color, as stones from different areas respond differently to this treatment.  Nearly all brown zircons from Sri Lanka will turn blue, while only a certain percentage of those stones which are mined in Thailand will turn out the same way.   Naturally yellow and red zircons from Tanzania do not treat to a blue color under this or any other treatment, although the yellow stones can be treated to become colorless.

Stones that do not treat to the desirable blue colors may be re-treated and altered by heating to about 900° centigrade in a furnace that is open to the air.  The results from this oxidizing heat treatment range from turning naturally colored stones to colorless, while some stones turn to a more general yellow orange color.  Naturally yellow and red zircons from Tanzania turned colorless as a result of this oxidizing version of the heat treatment.

Generally zircons are fairly stable toward heating in either type of process, but reasonable precautions should be taken to prevent the stones from being exposed to too excessive temperature shock to prevent cracking.