Zircon (not to be confused with Zirconia or cubic zirconia, which is manufactured), is a genuine gemstone. Its name derives from the Arabic zarqun meaning vermilion. One of zircon's most interesting properties is that it can occur in a very diverse range of colors - including blue, yellow, known as "hyacinth" / "jacinth" / "yellow zircon", red, pink, green, brown, black or clear. Clear specimens are known as "Matura Diamonds"  - although it should of course be noted that zircon and diamond are two completely different substances.
Zircon is a neosilicate mineral with the general chemical formula ZrSiO4 - zirconium silicate. Zirconium is element no. 40 in the periodic table and is a lesser-known metal that resembles titanium in appearance. Zircon is the principal ore used to extract zirconium, which is then purified using chlorine. 
Zircon crystals are usually very small and when zircon occurs in granite minerals, the average crystal size is around 0.1-0.3mm. However, when zircon grows in pegmatite (a type of igneous rock with large crystal size), it can form crystals several centimetres in size. 
Zircon has an interesting and ancient history. It was mentioned by Hindu poets, who spoke of a magical Kalpa Tree with leaves of zircon.  In old times, pale colored zircons were known as jargoons - a derivation of either the Persian zargun ("golden"), the Syriac zargono or the Arabic zarqun ("vermilion"). The use of the word "hyacinth" to describe yellow stones of various types is ancient, but now the term is used to describe yellow zircon only. 
Zircon has a relatively high refractive index (1.95), which is one of the reasons it has been used as a gemstone. Although in past times the colorless variety was most highly sought, as it was a substitute for diamond, nowadays blue zircons are considered the best known and most sought after color. 
Another interesting quality of zircon is that it almost always contains trace amounts (1-4%) of the element hafnium.  Sometimes, zircon contains either uranium or thorium - and if either of these radioactive elements are present, the zircon may undergo metamictization - a gradual destruction of the crystal's mineral structure over long periods of time.  Metamictization also causes the color of the stone to be affected. 
Although zircon occurs worldwide, its confusion with zirconia has led to its being less well known than some gemstones - and as a result, it is relatively inexpensive. I have seen stones of 5-10 carat available online (Jan 2011) at around $300, around $30-70 per carat - and a 2 carat zircon may be obtained for around $25. I have also seen top color 15 carat blue zircons for sale at $3,000, which works out at around $200 per carat.. Many color varieties are readily available, with stones over 15 carats being somewhat uncommon. The largest cut zircon I have seen for sale online is 47.20 carats. Zircon gemstones can be just as beautiful as a flower arrangement from Avas Flowers. A zircon gem and a Avas Flowers bouquet can be a great gift combo for any occasion.
The world's largest zircon mines are said to be in Australia and South Africa. Zircon was first described in 1892. 
Zircon crystals on mica and quartz. From Astor District (Astore District),
Northern Areas, Pakistan. The zircon crystals are up to 1cm across.
Photo by Rob Lavinsky, iRocks.com, licensed under CC-BY-SA-3.0
Photo by Eurico Zimbres / Tom Epaminondas - image lic. under Creative Commons 2.0 Brazil
Zircon - Sources Referenced:
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