Zircon is an unusual gemstone on a number of counts. It is the oldest mineral on Earth, with samples found in Australia that are more than 4.4 billion years old, older even than the moon. Zircon is of such interest to geoscientists that it has spawned a discipline of zirconology.
Another unusual thing about zircon is that its gemological properties exhibit very large ranges. For example, if you look at zircon's values for hardness, specific gravity (density) and refractive index, you'll notice the ranges are very wide. The gemological references 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.
Cyrstal Structure Zircon
The reason for the wide variation is that there are so-called low zircons and high zircons, with the high zircons having values at the top of range. The low zircons -- those with lower density, hardness and refractive index -- form as the result of a process associated with the presence of uranium and thorium in some zircons. The natural radioactivity disrupts the crystal structure and produces changes in color and density. These changes take place over a long period of geologic time through a process known as metamictization.
Natural Green Zircon
These so-called metamict zircons are always green or brown in color. Though externally they look much like normal zircons, internally they have lost their crystal structure and have become an 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 zircons are quite rare and are 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 possibly exist in other well-known zircon deposits in Cambodia.