About Green Zircon, Low Zircon & Metamict Zircon
Zircon is an unusual gemstone on a number of counts. Often confused with cubic zirconia, a made-made synthetic diamond substitute, natural zircon is actually the oldest mineral on Earth. In fact, according to samples found in Australia, researchers discovered that the material dates back from more than 4.4 billion years ago, even older than the moon itself. Zircon is of such important interest to geoscientists, it has spawned its own discipline known as 'zirconology'.
Another unusual thing about zircon is that its gemological properties exhibit very wide ranges. For example, if you look at zircon's values for hardness, specific gravity (density) and refractive index, you'll notice that the ranges are very wide. Gemological resources list zircon as having a hardness of 6.5 to 7.5; a density of 3.93 to 4.73; and a refractive index of 1.810 to 2.024.
The reason for the wide variation is that there is so-called low zircon and high zircon, with the high zircon having values at the top of the range. Low zircon - that with lower density, hardness and refractive index - forms as a result of a unique process associated with the presence of uranium and thorium. The natural radioactivity of these elements disrupts the crystal structure, thus causing changes in color and density. These changes take place over a long period of time through a process known as metamictization.
This so-called metamict zircon is always green or brown in color. Though externally it looks much like normal zircon, internally it has lost its crystal structure and has become isotropic glass. Though the metamictization process lowers a mineral's refractive index, hardness and specific gravity, it does not, curiously, affect the mineral's brilliance, dispersion or fire.
The green metamict zircon is quite rare and is prized by collectors. Some samples still emit low levels of radiation. The loss of crystal structure often makes the metamict specimens darker and cloudier, and some samples may be heated to lighten the color. Experiments have shown that prolonged heating can result in substantial recrystallization. Of all the many different varieties of colored stones available today, zircon is prized for its remarkable brilliance and fire.
Sri Lanka is the best known source for green metamict zircon, although low zircon specimens are also found in Burma, and may also exist in other well-known zircon deposits located throughout Cambodia. Cambodia and Southeast Asia are the important sources for fine zircon today, particularly when it comes to material suitable for the making of heat-treated blue zircon, which has become one of the most popular blue gemstones choices over recent years.