Zircon is an unusual gemstone on a number of counts. First, it is the oldest mineral on Earth, with samples found in Australia that are more than 4.4 billion years old, even older than the moon. Zircon is of such interest to geoscientists that it has spawned its own discipline; 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. The low zircon - that with lower density, hardness and refractive index - forms as a result of a process associated with the presence of uranium and thorium. The natural radioactivity disrupts the crystal structure and causes 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 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.
Sri Lanka is the best known source for green metamict zircon. Specimens are also found in Burma, and may exist in other well-known zircon deposits located in Cambodia.
- First Published: November-13-2009
- Last Updated: August-04-2014
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