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Zircon is a mineral remarkable among those employed as gems for its high specific gravity and adamantine luster. For these reasons the colorless, transparent stones are sometimes employed as substitutes for the diamond, although they lack the high refractive power and hence play of colors of the latter. The stones are sometimes called "Matura diamonds," because of their abundance at Matura in the island of Ceylon. The colorless, or smoky zircons, are often known as " jargons" or " jargoons," a name said to have been given in allusion to the fact that though they resembled the diamond in luster they had really much less value. Besides zircons of this sort there are those known in jewelry as " hyacinth " or " jacinth," which are transparent zircons of a brownish, red-orange color. A stone of a nearly similar color is furnished by the essonite variety of garnet, and this is also often known as hvacinth.
The high specific gravity of zircon above referred to is more than four times the weight of water, determinations giving results varying between 4.2 and 4.86. Zircon is thus the heaviest of gems, and will sink at once in any of the ordinary heavy liquids. The hardness of zircon is between that of quartz and topaz, being 1\. Its index of refraction is high, being 1.92, or near to the diamond among gems, a fact which accounts for its brilliancy when cut. Before the blowpipe zircon is infusible. It is not acted upon by acids except in fine powder by sulphuric acid. In composition it is a silicate of zirconium, the percentages being silica 32.8, zirconia 67.2. It usually also contains a little iron oxide. It is not an uncommon mineral in rocks, occurring in crystals of microscopic size, and in crystalline rocks it sometimes occurs in large and abundant crystals. These are usually opaque and of no value for gem purposes, although they are mined to some extent at the present time for use in incandescent lights. Opaque zircon is found in this country in Georgia, Colorado, New York, and Canada. The form of the crystals is usually that of four-sided prisms terminated by pyramids. The transparent zircons available for gems, that is, the so-called " noble " zircons, come almost wholly from the island of Ceylon, where they occur in the gem gravels that contain also rubies, sapphires,
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