Zircon

Category  :  Mineral
Hardness  :  7.5
Gravity  :  4.6–4.7
Reflection  :  2.01 / 1.96
Color  :  brown, red, yellow, green, black, and colorless
Chemical formula  :  ZrSiO4
Crystal system  :  Tetragonal
Major varieties  :  Colourless Zircon, Blue Zircon, Red Zircon, Yellow Zircon, Brown Zircon, Green Zircon

Hafnium is almost always present in quantities ranging from 1 to 4%. The crystal structure of zircon is tetragonal crystal system. The natural color of zircon varies between colorless, yellow-golden, red, brown, and green. Colorless specimens that show gem quality are a popular substitute for diamond; these specimens are also known as "Matura diamond". It is not to be confused with cubic zirconia, a synthetic substance with a completely different chemical composition.

The name either derives from the Arabic word zarqun, meaning vermilion, or from the Persian zargun, meaning golden-colored. These words are corrupted into "jargoon", a term applied to light-colored zircons. Yellow zircon is called "hyacinth", from the flower hyacinthus, whose name is of Ancient Greek origin; in the Middle Ages all yellow stones of East Indian origin were called hyacinth, but today this term is restricted to the yellow zircons.

Zircon is a remarkable mineral, if only for its almost ubiquitous presence in the crust of Earth. It occurs in igneous rocks (as primary crystallization products), in metamorphic rocks and in sedimentary rocks (as detrital grains). Large zircon crystals are seldom abundant. Their average size, e.g. in granite rocks, is about 100–300 µm, but they can also grow to sizes of several centimeters (a few inches), especially in pegmatites.

Colourless Zircon

The name zircon has become synonymous with cheap imitation, hopelessly discrediting even the beautiful, colored specimens. Colorless zircon can, apparently, also be obtained by heat-treatment of brown or reddish stones. It is mainly given a brilliant cut, but often a slightly different type, with eight extra facets added to the pavilion, starting from the tip, to improve its luster (this is in fact known as the zircon cut).

Blue Zircon

The best color is a light electric blue not found in any other gem, with pleochroism making it look greenish in one direction. It is often given a zircon-type brilliant cut, which is not always round; also used are rectangular or square, step cuts. Mixed cuts are less frequent. This gem is not manufactured synthetically.

Red Zircon

Like all zircons, it has considerable luster, shown to advantage by the zircon cut (a modified for m of brilliant cut) sometimes used for these stones. As always with zircons, the marked birefringence is useful means of recognition. It has the physical properties of high zircon, with refractive indices above the range of normal refractometers. It has a highly distinctive absorption spectrum.

Yellow Zircon

The color may vary from a rather pale yellow to canary yellow, gold or greenish yellow. It is most often given a round or oval, mixed cut. It has delicate, brilliant edges. Strong birefringence, easily seen with a lens, will readily distinguish it from yellow sapphire, which it can resemble, and from artificial products, such a YAG and cubic zirconia, which may also be yellow, but are single refractive.

Brown Zircon

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. Oval or round mixed cuts are the most common and the edges are often a bit chipped, as frequently happens in zircon. The luster and marked birefringence are highly characteristic and easily established.

Green Zircon

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 main signs of breakdown of the crystal lattice. The luster is affected by this turbidity and is often much less strong than in other zircons.

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