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: Zircon

Zircon Gemstones used in Jewelry

Rough & Polished Zircon Crystals

Source: Brazil, Cambodia, Canada, France, India, Italy, Madagascar, Norway, Russia, Thailand, South Africa, Sri Lanka, USA

Birthstone: December (Alternate: Tanzanite)

The name "zircon" (zirkon German, circon Spanish, zircone Italian, zirc‹o Portuguese) is derived from the Arabic word zarqun, meaning "vermilion," and from the Persian word zargun, meaning "golden-colored." Zircon should not be confused with "cubic zirconia," which is a man-made diamond simulant that is in no way chemically similar to this ubiquitous mineral.

Zircon (zirconium silicate) is a common mineral that belongings to the group of nesosilicates, or basic silicate rock-forming minerals. Zircon is found in association with alkaline volcanic rock (felsic and granite pegmatites), metamorphic rock, sedimentary rock, as well as within secondary deposits of gem-bearing gravels. Zircon is also a common constituent in mineral sands.

The crystal structure this uniaxial mineral is tetragonal, and its crystal habit is dipyramidal and prismatic, with crystals often being twinned.

Zircon Crystal Model

Zircon Crystal Model

   Zircon Dust

Zircon Dust (USGS)

Zircons can be mildly radioactive, containing trace amounts of the chemical elements uranium (U), thorium (Th), and the transition metal hafnium (Hf) is typically present in zircon. Using radiometric dating, zircons found in Western Australia's Yilgarn Craton and Narryer Gneiss Terrane are some of the oldest minerals found on earth, at 4.4 billion years.

Zircon Color & Refraction

Zircon occurs in a wide varieties of colors which include: brown, orange-red or yellow (jacinth), red or violet-red (hyacinth), as well as green, blue, and black. There is also a colorless zircon called jargoon, which can be mistaken for diamond. Sri Lanka has a clear variety of zircon that is referred to as "Matara diamond," and is a popular, yet inexpensive diamond substitute.

What makes zircon particularly appealing as a faceted gemstone is its high refractive index (1.90 average, compared to diamond's refractive index of 2.4), and its diamond-like adamantine luster, yet this is where the similarity with diamonds diverges. Zircon has a hardness of only 7.5 on the Moh's scale, and the toughness, or "tenacity" of zircon is only rated as fair to good. It is for this reason that care must be taken to avoid chipping or abrading the facet edges.

Zircon Crystal Structure

Unlike diamond, zircon is both doubly refractive and anisotropic, with varying refractive indices along its a or b crystallographic axes. Zircon is also one of the heaviest gemstones, resulting in smaller sizes (measured in milimeters) for a given carat weight.

Zircon Crystallography, Chemistry, Physical Properties

Crystal System tetragonal
Crystal Habit dipyramidal prismatic
Specific gravity (SG) 3.9 to 4.7
Mohs Hardness Scale 6.5 to 7.0
Toughness fair to good (brittle)
Fracture subconchoidal to uneven
Cleavage indistinct in two directions, poor
Streak white
Chemical Composition ZrSiO4

Zircon Optical Properties

Optical Properties uniaxial, doubly refractive
Refractive Index 1.78-1.85 (a-axis), 1.92-2.01 (b-axis)
Birefringence 0.0-0.008 (a), 0.036-0.060 (b)
Pleochroism pleochroic haloes around zircon inclusions
Surface Luster vitreous to adamantine
Fluorescence yes
Diaphaneity transparent to opaque
Gem Color black, blue, brown, green, orange, red, yellow

Transparent zircon can be found in shades of brilliant green, brownish-green and yellow-green. These stones typically have unique inclusions known as "spangles," as well as streaks and/or angular color zoning. Yellow zircon can be mistaken for yellow sapphire, and can be found in the canary yellow, greenish yellow, and golden-yellow. Perhaps the most popular gemstone color for zircon is blue.

Blue Zircon

Blue Zircon (photo: © AfricaGems.com)

   Sri Lanka Green Zircon

Green zircon from Sri Lanka (photo: © AfricaGems.com)

Zircon Color Enhancements

Zircon can be heat treated to alter its color, and varying temperatures can produce colorless, blue, or yellow gems from less desirable specimens. Heat treatment of reddish-brown stones found in Cambodia, Thailand, and Vietnam produce most of the blue zircon gemstones on the market, but many of the red, red-orange, or violet-red zircon on the market has not undergone any kind of heat treatment. The resulting color change of heat-treated zircon is permanent. To date, there are no examples of a synthetic zircon.

Zircon Mining

The principal sources for zircon are found in Cambodia, Thailand, and Sri Lanka. Colorless varieties of zircon from Matara, Sri Lanka has become synonymous with fake diamonds, and have come to be known as "matara diamonds." There is also a chatoyant variety of zircon found in Sri Lanka.

Zircon is mined for its zirconium, which is used as a compound in abrasives, and it is also the source for zirconium oxide which is one of the most refractory (high-temperature, high-strength) materials known, used in high-temperature crucibles and nuclear reactors [3].

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Bibliography and Reference on Zircon

1. Wuerzburg Uniaxial Minerals - Zircon . www.geographie.uni-wuerzburg.de

2. Edna B. Anthony The Zircon Group - A Nesosilicate . www.attawaygems.com

3. Stephen A. Nelson Introduction to Uniaxial Minerals . www.tulane.edu

5. Judith Crowe, The Jeweler's Directory of Gemstones . DK Publishing.

6. Walter Schumann, Gemstones of the World . NAG Press; 2Rev Ed edition

7. Renee Newman, Gemstone Buying Guide . International Jewelry Publications; 2nd edition

8. Africa Gems, Cut Zircon . www.africagems.com

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