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Zircon

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Blue zircon is one of the traditional birthstones for the month of December.

 Zircon is one of the few stones to approach diamond in fire and brilliancy. Zircon is a superb gem due to its high refractive index and color dispersion. Its colorless stones can closely resemble diamonds, and they have been intentionally and mistakenly substituted for them. Zircon exhibits double refraction, and bottom facets are seen through the top of a cut stone will appear double. Diamond does not share this property, and so this is a useful test for distinguishing between the two gemstones; zircon is also significantly softer than diamond, and cut stones will show wear on the edges of the facets. 

 

Zircon has been known since antiquity, and takes its name from the Arabic zaragun, derived in turn from the Persian zar, meaning "gold", and gun, meaning "color". It has been mined for over 2000 years from the gem gravels of Sri Lanka and was used as a gemstone in Greece and Italy as far back as the 6th century AD. It forms prismatic to dipyramidal crystals, which can be colorless, yellow, gray, green, brown, blue, and red. Single crystals can reach a considerable size: examples weighing up to 5 lb (2 kg) and 10 lb (4 kg) have been found in Australia and Russia, respectively. Its colored varieties have been given other names in the past, although these are now obsolete: the transparent red variety has been called hyacinth (jacinth); clear and colorless zircon from Sri Lanka has been called Matura diamond; and the name "jargon" or "jargoon", derived from the Arabic zargun, has been applied to all other gem colors. Green zircon represents the green foliage of the Kalpa Tree, a gemstone that is a symbolic Hindu offering to the gods. Elsewhere in the East, wearing zircon was believed to endow the wearer with wisdom, honor, and riches. It was also an amulet for travelers. 

                              

Zircon is a silicate mineral, zirconium silicate, (Formula: Zr(SiO4) with a tetragonal crystal system, and is the principal source of the rare mineral zirconium, a highly corrosion-resistant metal used to coat the interiors of nuclear reactors. Zircon crystals almost always contain traces of the radioactive elements hafnium, uranium, and thorium. These eventually break down and destroy the crystal structure, which can often be restored by heat-treating the crystal. Heat-treating minerals to change their color or improve their clarity has been a widespread practice for centuries, and it may well have begun with zircon. Brown stones from Thailand and Sri Lanka turn blue or colorless when heated; in blue stones that have lost their color, reheating will establish the blue color. The process itself does not require expensive apparatus. The zircons to be treated are placed in a clay pot, intermixed with fine sand; the pot is heated in the coals of a campfire for a period determined by long experience. After slow cooling, the newly colored stones are separated from the sand and cut. Blue zircon reheated in the presence of oxygen changes to a golden-yellow, the source of the name zargun. Because the word is of ancient origin, so, too, must be the practice of heat-treating. 

Zircon is also useful because it has a high melting point and is therefore used to make foundry sand, heat-resistant materials, and ceramics. Because of its hardness (), zircon is also used to make industrial abrasives. 

Zircon is widespread as a minor constituent in silica-rich igneous rocks; it also occurs in metamorphic rocks. It is resistant to weathering, and because of its relatively high specific gravity (4.6 - 4.7), it concentrates in stream and river gravels, and in beach deposits. Gem varieties are mostly recovered from stream gravels in Sri Lanka, Thailand, Myanmar, Australia, and New Zealand. Beach sands are a major source of commercial production, particularly in India, Australia, Brazil, and Florida. Other key localities are Canada, Mexico, Norway, France, and Pakistan. 

In the Middle Ages, this gem was thought to induce sound sleep, drive away evil spirits, and promote riches, honor, and wisdom.  Blue zircon was popular in the Victorian Era, and colorless zircon was used in the early 1900’s as a diamond substitute.  Celebrated gemologist George Kunz was a big fan of zircon and tried to market it under the name “starlite” to express its fiery qualities.  However the nickname never caught on, and zircon stayed out of the mainstream jewelry market.  

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