Zircon is a birthstone for the month of December, along with turquoise and tanzanite. A Zirconium silicate mineral (ZrSiO4), It is often found with some hafnium and occasionally with some uranium, thorium, and yttrium. This silicate can contain up to 20 percent of hafnium in its structure; if it exceeds that amount then it is called Hafnon.
Zircon is a well-known mineral and an important gemstone of many colors. Its brilliant luster and fire, combined with good hardness, make it a desirable gem. Natural Zircon with good color and transparency is rare. Most crystals are opaque and brownish. Other natural colors are black, gray, light brown, brownish-red, orange, pink, yellow, light blue, light green, light purple, white, and colorless depending on the trace elements found in the specimen.
However, most Zircon gemstones are enhanced. Heating and irradiation can be used to produce colorless, blue, green, and many other zircon colors. Blue is the most popular zircon color. About 80% of the zircons sold today are blue. Zircon has been used as a gemstone for over 2000 years. Its very high dispersion and refractive index give it a brilliance and fire that rival those of diamond. For that reason, colorless faceted zircon has been used as both a popular and fraudulent substitute for diamond.
Physical Properties of Zircon
This beauty belongs to the Tetragonal Crystal System, Ditetragonal dipyramidal Crystal Class. Hardness is 7.5 on the Moh’s Scale. Specific Gravity is 4.6 – 4.8. Luster is greasy to adamantine. Crystal forms are usually short and stubby crystals, as well as prismatic which are sometimes elongated. Crystals are almost always terminated with a pyramidal termination. Crystals may be doubly terminated, and occasionally entirely pyramidal resembling an octahedron. Also grainy, as fibrous aggregates, and as rounded, water worn pebbles. Twinned crystals are uncommon but do exist. It is found in igneous, metamorphic and sedimentary environments. Some may be fluorescent orange-yellow in shortwave ultraviolet light.
Radiometric Dating: Zircon has played an important role during the evolution of radiometric dating. Zircons contain trace amounts of uranium and thorium and can be dated using several modern analytical techniques. Because zircons can survive geologic processes like erosion, transport, even high-grade metamorphism, they contain a rich and varied record of geological processes. Currently, zircons are typically dated by uranium-lead (U-Pb), fission-track, cathodoluminescence, and U+Th/He techniques. Zircons in sedimentary rock can identify the sediment source. Zircons from Jack Hills in the Narryer Gneiss Terrane, Yilgarn Craton, Western Australia, have yielded U-Pb ages up to 4.404 billion years, interpreted to be the age of crystallization, making them the oldest minerals so far dated on Earth.
Noteworthy Localities of Zircon:
Many good localities for Zircon are known worldwide. In Russia, excellent Zircon specimens come from the Lovozero Massif in the Kola Peninsula; and at Vishnevye in the Ural Mountains. Sharp, lustrous brownish-red crystals come from Pakistan in Gilgit, Chilas, and Harchu; and crystals of similar quality from Darra-i-Pech, Nangarhar Province, Afghanistan. Transparent gemmy green and brown Zircon comes from Mogok, Myanmar; and large elongated crystals from the Giant Crystal Quarry, Ratnapura, Sri Lanka.
Important European Zircon localities include Store Kufjord, Seiland Island, Norway (famous for its fabulous transparent dark red crystals); the Nibbio mine, Mergozzo, Piedmont, Italy; the alluvials of Rochefort-Montagne, Puy-de-Dôme, Auvergne, France; and the Laach lake volcanic complex, Eifel Mts, Germany (as strangely colored white and light-yellow crystals).
Large, dull crystals come from Mud Tank in the Harts Ranges, Northern Territory, Australia; and short, stubby pyramidal crystals from Peixe, Goias, Brazil. Outstanding cream-colored Zircon crystals come from Mount Malosa, Zomba Region, Malawi; and large crystals have come from several of the Madagascar pegmatites, especially in the Amboasary District, Tuléar Province.
In Canada, one of the most exceptional Zircon localities is the Bancroft District, Hastings County, Ontario (especially in Dungannon and Lake Clear Townships). Large, well-formed crystals are famous from Tory Hill, Wilberforce, Haliburton Co., Ontario. Lustrous sharp crystals come from the famous Poudrette quarry, Mont Saint-Hilaire, Québec; and large crystals from the Kipawa Alkaline complex, Lac Sheffield, Témiscamingue, Québec.
In the United States, one of the most important localities is the Eureka Tunnel, St. Peters Dome, Cheyenne District, El Paso County, Colorado. Very good Zircon crystals come from Pacoima Canyon in the San Gabriel Mts, Los Angeles County, California; and an old Zircon locality that once produced fine large crystals is the Wichita National Wildlife Refuge near Indiahoma, Comanche County, Oklahoma. Doubly terminated gray floater crystals come from Zirconia and Tuxedo, Henderson County, North Carolina. Massive radioactive Zircon comes from Spruce Pine, Mitchell County, North Carolina; and large sharp crystals from the Tigerville Prospect, Greenville County, South Carolina.
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Article by Bill Jones, Sidewinder Minerals