Zircon is a mineral with a hardness of 8 out of 10 on the Mohs scale of mineral hardness [?]. These Tetragonally structured gems are made of zirconium silicate, their full chemical compound being ZrSiO4.
Zircon is a zirconium silicate with a tetragonal crystal system, and also contains thorium and uranium.
It appears as stubby, prismatic, isolated or sometimes dipyramidal, like two pyramids connected at their bases; or in twins, colorless to yellow, red, brown, gray or green, in irregular granules.
It is very hard, heavy, with indistinct cleavage, and shell-like fracture. It is sometimes perfectly transparent with luster like diamond and strong birefringence. But it has a particular brittleness, which is shown when the edges on faceted stones are rubbed.
It may also be opaque, dull, and almost amorphous when its structure is destroyed by radioactive thorium and uranium, which substitute for up to 4 percent of the zirconium, and break down the structure of the crystal. This is called metamict zircon. But this does not affect the varieties of zircon used as gems.
Zircon as inclusions has been seen in sapphires, tourmalines, garnets, spinels, etc.
It is a typical accessory mineral of acidic igneous rocks and their metamorphic derivatives. It is generally formed in intrusive magmatic rocks and is also found in the pegmatites derived from them, except in metamorphic schists. It is also concentrated in alluvial deposits in the form of small grains, being fairly resistant to the elements.
It occurs in beach sands of, and in the Mud Tank area of the Harts Range, Northern Territory in Australia, Brazil, Florida (USA), and in Nigeria. Splendid crystals are found in alluvial deposits in Matura (Sri Lanka) and in gold gravels in the Urals (Russia). Large crystals have been found at Renfrew (Canada), Litchfield, Maine (USA), in northern Norway, and Sweden. The most famous sources of gem quality zircon are in the gem gravels of Sri Lanka, Myanmar, Cambodia, Vietnam, and Thailand.
The varieties of zircon include:
This is the variety best known to the public, for years regarded as a substitute for diamond, and most people could not tell the difference. Because of this the name 'zircon' has been taken to mean 'cheap imitation,' unfortunately for the beautiful, colored specimens of genuine zircon. Colorless zircon can also be apparently obtained by heating of brown or reddish stones. This is regarded as a normal, and not a fraudulent practice.
The heated stones in a closed furnace turn blue or colorless. By passing air through the furnace, the color changes to red or golden yellow. Once a stone's color is changed, it rarely reverts to its original color.
Colorless zircon is normally given a brilliant cut, but often of a slightly different type, with eight extra facets added to the pavilion, to improve its luster. Because it is somewhat brittle, it is not very suitable for rings, as the stones can easily lose their polish.
Even if it has fallen out of demand as a substitute for diamond, zircon is still imitated, but not produced synthetically. Colorless synthetic spinel, YAG (yttrium aluminum garnet), and cubic zirconia appear on the market under the name of zircon.
Blue zircon (also known as starlite)
This variety has not been known for long, partly because it is often, but not always, obtained by heat treatment of other colored zircons. The best color (not often seen) is a light, electric blue not found in any other gem. It can also be sky blue with less obvious pleochroism, a very soft, pale blue, or a distinctly light greenish blue. The best quality, electric-blue stones, are uncommon, but not in great demand because they are not widely known. They are worth slightly less than the top level secondary gems.
This variety was once known as hyacinth, although it did not match the description of the hyacinth written about by Pliny the Elder. A number of orange-red or brownish red stones were called by this name until the knowledge of mineralogy in the eighteenth century caused these names based purely on color to be abandoned.
The red zircon is often a brick- or orange-red, but sometimes even a violet red. It is more or less equal in cost to the best blue zircons.
This mineral was known in the past as "jargon", and the name evolved the word "zircon" now applied to the entire mineral species. The color may vary from a rather pale yellow to canary yellow, gold, or greenish yellow. It has the typical striking luster of zircon. Its value is somewhat modest, lower than that of the blue and red varieties.
This variety occurs in abundance, but probably the least appreciated of all the zircons. It is a brown color, which can vary from "black tea" to reddish brown, tobacco, or yellowish brown, all of which are uncommon in other gems. Its luster is as fine as that of other zircons.
A relatively common stone, but may vary a great deal in its characteristics, the green color generally being associated with the metamict state, where thorium and uranium have broken down the zircon crystal structure. The color varies from a slightly brownish green to brilliant, rather cold green, or yellow green. Some examples are perfectly transparent; others can look cloudy and display close, parallel striations, which are the main signs of the breakdown of the crystal lattice. Its luster is affected by this so is often much less strong than that of other zircons.
Zircon may have derived its name from the Arabic "zarkun" meaning "red" or more probably from the Persian "zargun" meaning "golden yellow" thereby giving its name to "jargon," the yellow variety of zircon. Zircon was also called war bride's diamond, because during the 1940s, natural diamonds were scarce.
Zircon is an important ore for zirconium, hafnium, and thorium.
Gem-quality zircon is known as "high zircon" or "alpha zircon". This is zircon not subjected to decomposition, and has the highest densities, refractive indices, and strong birefringence. It is considered a very important gemstone, which has high brilliance and dispersion hence the cut stones have a good fire. Due to the large double refraction of zircon, the stones are cut so that minimum fuzziness or reflection can be achieved.