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Zircon
 Zircon was well known to ancient civilizations and its use as a gem extends back at least to Greek and Roman times. As usual it went under different names according to its color varieties and was often confused with other species. One of its names was 'hyacinth' (taken from Greek mythology), which later became 'jacinth', and normally applied to the magnificent reddish-brown varieties of zircon, but also to hessonite garnet. Another name, 'jargoon', probably of Persian or Arabic derivation, was applied to pale yellow zircons.Zircon, a silicate of the metal zirconium, occurs quite commonly in igneous rocks such as granite. There are only a few areas where zircons of gem quality are found. Ceylon, that treasure-house of gems, ranks high among these and produces stones of many colors but perhaps the finest zircons come from Cambodia from where they are taken to Bangkok in Thailand for processing and distribution. The natural colors of zircon range from colorless (rare) through all shades of yellow and brown to deep brownish-red or a brownish-green. The darker colors can be improved by heat treatment owing to a most unusual peculiarity of zircon.  Most zircons contain varying amounts of uranium which emits neutrons with a great force. This collide in the surrounding zircon destroys its crystallinity thus greatly effecting its physical properties. It is therefore possible to divide zircon in three types: a 'high' type in which there is little or no damage to the crystal structure; an 'intermediate' type in which the crystal structure has been disorganized to a greater or lesser degree; and a 'low' type in which the crystal lattice has become completely destroyed. The less crystalline a zircon is, the lower its hardness, its refractive index and density, its birefringence, its dichroism and the clarity of its very distinctive and interesting absorption spectrum. Color changes accompany the changes in crystal structure; low stones are green, or extremely rarely orange. Most stones when found are intermediate and may be of any of the colors named. High zircon has a hard, clear sparkle, an adamantine lustre and a high dispersion of color. It is hard but somewhat brittle, so that its facet edges are apt to show wear. Its birefringence is so strong that in a large stone the back facets appear double even to the naked eye. It has the highest density of any gemstone in common use and is normally perfectly clear and transparent with an extensive range of fine stones. These are surely qualities that should procure for jewelry an even wider popularity than it possesses. The high lustre of zircon makes the brilliant cut easily the most appropriate style of cutting this gem. As a ring stone it should be worn with care but in all other settings it enhances jewelry with its brightness and fire. The most highly esteemed colors are a rich blue or red, followed by golden shades. Colorless zircons are favorite substitutes for diamond. Zircon of different colors, often grouped together in the same setting with less bright stones, do not always harmonize well with them. There is need to mention that most people bumble cubic zirconia, the laboratory-grown diamond imitation with a natural gemstone called zircon. Blue zircons are sometimes imitated in synthetic spinel or corundum, but no synthetic zircon is on the market. Some near-white zircons from Matara in Ceylon are marketed under the misleading name of 'Matara diamond'. 
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