Zircon

zircon
Zircon rough

Zircon is most famous for its colorless stones, which closely resemble diamonds and have been used both intentionally and mistakenly in their place. Although colorless when pure, impurities will produce yellow, orange, blue, red, brown and green varieties. Brown stones from Thailand, Vietnam and Kampuchea are usually heat-treated to change them into the colorless or blue stones popular in jewelery. Blue stones that revert to brown will regain the blue if reheated. Blue zircon reheated in the presence of oxygen will change to golden-yellow. Zircon may be distinguished from diamond by its double refraction and by wear and tear on its facet edges. It has been imitated by both colorless glass and synthetic spinel. Some zircon contains radioactive thorium and uranium, which eventually break down the crystal structure. Decayed stones are known as “low” zircon, with a “metamict” structure; undamaged material is “high” zircon.

Gem-quality crystals are usually found as pebbles in alluvial deposits. Sri Lanka has been a source of gem material for over 2,000 years; other localities include Burma, Thailand, Cambodia, Vietnam, Kampuchea, Australia, Brazil, Nigeria, Tanzania, and France.

Zircon was believed to provide the wearer with wisdom, honor, and riches and loss of lustre was said to warn of danger. The name is from the Arabic zargun, which derives from the Persian for “gold color”.

Cause of Color :

Color centers.

Chemical Composition :

Silicate of Zirconium with traces of Uranium and Thorium – ZrSiO4

Classification / Type :

Classified into 3 categories – high, medium and low type. The presence of radioactive elements within the structure may cause the breakdown of the internal structure (metamictization). The extent of breakdown defines the three types.

Crystal System / Forms :

Tetragonal System / Prismatic habit and also as water-worn pebbles. Tetragonal prisms with pyramidal and / or pinacoidal terminations.

Cuts & Uses :

Facetted cuts, cabochons, beads, etc.

Dispersion : Weak to Moderate. Maximum: 0.038
Hardness : 6.5 - 7.5 (low to high type)
Lustre : Adamantine to vitreous.
Magnification :

Angular zoning and streaks (are sometimes seen in low type), crystals, doubling of facet edges, abraded facet edges.

Optic Character :

Anisotropic, D.R.; Uniaxial Positive. Low type metamict zircon maybe almost amorphous.

Pleochroism : Variable dichroism with blue zircon showing the strongest effect.
Refractive Index / Birefringence :
  • High: 1.925 – 1.984 / 0.059
  • Medium: 1.875 – 1.945 / 0.006 – 0.050
  • Low: 1.810 – 1.815 / 0.002 – 0.005
Simulants (with separation tests) :

Sphene (optic figure, dispersion, spectrum), Synthetic Cubic Zirconia (optic character, dispersion, spectrum), Y.A.G. (optic character, spectrum), G.G.G. (optic character, spectrum, S.G.), Strontium Titanate (optic character, S.G. dispersion), Diamond (optic character, S.G., spectrum), Demantoid (optic character, spectrum, inclusions), Malaya Garnet (optic character, S.G., spectrum), Topaz (R.I., S.G., spectrum, dispersion), Natural / Synthetic Sapphire (R.I., spectrum, doubling), etc.

Sources :

Sri Lanka, Myanmar, Thailand, Cambodia, France, India, Australia, Madagascar.

Specific Tests :

Metamict Zircon: Low type zircons in which the internal structure is broken down due to residual radioactive decay. Though doubly refracting, they become almost singly refracting. Their properties are lower (R.I., S.G.) and they may exhibit weak doubling.

Spectrum :
  • The strongest line is at 653.5nm due to Uranium.
  • Many narrow lines and strong bands across the whole spectrum, ranging from more than 40 lines (in Myanmar Zircon) to only a few lines (in Australian Zircon). Other lines are 653.5nm, 691.0nm, 683.0nm, 662.5nm, 660.5nm, 621.0nm, 615.0nm, 589.5nm, 562.5nm, 537.5nm, 516.0nm, 484.0nm, 460.0nm, 432.7nm
  • Heat treated and low type zircon has a weak spectrum.
  • Colorless, blue and golden brown (all heat treated) zircons exhibit one fine line at 653.5nm
  • Some of zircons may display no spectrum at all.

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