Transparent zircon is a well-known form of semi-precious
gemstone, favored for its high
specific gravity (between 4.2 and 4.86) and adamantine
luster. Because of its high
refractive index (1.92) it has sometimes been used as a substitute for
diamond, though it does not display quite the same
play of color as a diamond. Zircon is one of the heaviest types of gemstone. Its
Mohs hardness is between that of
quartz and topaz, at 7.5 on the 10 point scale, though below that of the similar manmade stone
cubic zirconia (8-8.5). Zircons may sometimes lose their inherent color after long exposure to bright sunlight, which is unusual in a gemstone. It is immune to acid attack except by
sulfuric acid and then only when ground into a fine powder. Most gem-grade zircons show a high degree of
birefringence which, on stones cut with a table and pavilion cuts (i.e., nearly all cut stones), can be seen as the apparent doubling-up of the latter when viewed through the former, and this characteristic can be used to distinguish them from diamonds and cubic zirconias (CZ) as well as soda-lime glass, none of which show this characteristic. However, some zircons from Sri Lanka display only weak or no birefringence at all, and some other Sri Lanka stones may show clear birefringence in one place and little or none in another part of the same cut stone. Other gemstones also display birefringence, so while the presence of this characteristic may help distinguish a given zircon from a diamond or a CZ, it will not help distinguish it from, for example, a
topaz gemstone. The high specific gravity of zircon, however, can usually separate it from any other gem and is simple to test. Also, birefringence depends on the cut of the stone in relation to its
optical axis. If a zircon is cut with this axis perpendicular to its table, birefringence may be reduced to undetectable levels unless viewed with a jeweler's
loupe or other magnifying optics. The highest grade zircons are cut to minimize birefringence. The value of a zircon gem depends largely on its color, clarity, and size. Prior to World War II, blue zircons (the most valuable color) were available from many gemstone suppliers in sizes between 15 and 25 carats; since then, stones even as large as 10 carats have become very scarce, especially in the most desirable color varieties. They are occasionally used in jewellery such as earrings. Zircons are sometimes imitated by
spinel and synthetic
sapphire, but are not difficult to distinguish from them with simple tools. Zircon from
Ratanakiri province in Cambodia is heat treated to produce blue zircon gemstones, sometimes referred to by the trade name
cambolite. In 2024, the Australian gemstone firm NYBlue announced their plans to establish control over the global blue zircon market by securing a substantial portion of the world's supply. The company reported an inventory exceeding one million carats of raw and faceted stones, citing recent mine closures in Cambodia as a factor in the gemstone's increasing scarcity. ==Occurrence==