ZIRCON    Blue Zircon

   Zircon is zirconium silicate, ZrSiO4, and is notable as a gem mineral for its high R.I., high dispersion, and subadamantine luster. Prior to advent of modern gem synthetics, colorless zircon was the stone of choice as a diamond simulant because of its diamond-like optical properties. Cubic Zirconia is not synthetic zircon but is zirconium oxide (ZrO2). Nearly all zircon contains trace quantities of U, Th, and Pb. The natural radioactive decay of uranium and thorium within zircon damages the crystal lattice, sometimes to the extent that crystals become virtually amorphous, and results in an atomic structural state that is referred to as "metamict". Such damage can render crystals darker and cloudier, and lowers the measurable optical and physical properties.

    Zircon is present in trace quantities in nearly all intrusive igneous rocks, particularly granites, where it occurs as very small crystals. Gem zircon is nearly restricted in occurrence to pegmatites or to quartz-poor, alkalic intrusive rock (e.g. syenites). The gem gravels of Thailand are the most important commercial source, followed by production from gem gravels in Myanmar and Sri Lanka.

    Gem-quality zircon is normally brown, but natural blues, oranges, greens, yellows and colorless stones are known. Color is the product of color centers and/or trace quantities of Fe. Nearly any color of the rainbow can and is produced by heat treatment of brown zircon. Heat treatment of zircon is ubiquitous.

Properties

  • Crystal System: Tetragonal
  • Crystal Habit: Massive, or as well formed tetragonal prism with pyramidal termination
  • Cleavage: 1, indistinct
  • Fracture: conchoidal
  • Hardness: 6.5-7.5; gem material usually 7-7.5.
  • Toughness: fair to poor; can be quite brittle
  • Specific Gravity: 3.95-4.8 (4.7)
  • R.I.: high, commonly 1.92-1.98. Reported range 1.78 -2.01.
  • Birefringence: High; 0.04-0.06
  • Dispersion: Strong; 0.039
  • Pleochroism: Strong in deep colors; Blue: deep sky blue to colorless or yellowish-Grey; Others: darker and lighter tones of the same color.
  • U.V. Fluorescence: Some show a mustard-yellow or yellow orange under long or short wavelength U.V.. Some show a dull yellow phosphorescence. Prolonged exposure to strong u.v. light can cause some colorless zircons to revert to their original brown color.

Distinguishing Properties

  • From diamond and c.z. with a polariscope, and by marked birefringence (doubling of pavilion facets).
  • From most others by high R.I. (off scale of refractometer)
  • Inclusions (rutile, Fe-oxides) characteristic, as is color zoning

Sources

    Thailand (most of worlds present production), Sri Lanka, Burma, Cambodia, Laos and Vietnam. All from gem gravel deposits or gem alluvium. Produced as a byproduct of corundum mining.

Shaping and Treatment

  • Usually cut in round brilliants to take advantage of high dispersion and R.I.. Zircon is a brittle material and tends to chip easily, particularly along facet junctions.
  • Heat treatment customarily applied (usually at mine site, prior to cutting) to alter color to desirable golden yellow, blue, and colorless. Heated for a few hours in reducing environment (surrounded by charcoal) at about 1000o C. Some (about 30%?) browns turn blue or colorless, remainder develop dark patches. Repeat treatment to patchy stones at about 900oC, in oxidizing environment to get colorless, yellow, orange or red. Unaffected stones can be recycled until yielding proper color. Treatment usually yields colorfast stones, but not always. Some stones known to fade in sunlight, or change color when exposed to strong u.v. or infrared light (e.g. tanning booths!). Obtain written guarantee when purchasing.
  • Sri Lankan dull green zircon is apparently heated to a dull red color for an hour which lightens the color. Reddish brown zircon treated the same way often yields colorless stones.

Pricing and Valuation

  • "Electric" blue stones are the most prized (top quality about $200/ct.) followed by red, orange and yellow.
  • "Commercial" blues are considerably less (ca. $35/ct for 2-3 carat stones). Mixed hue stones are worth considerably less than pure colors. Clean stones larger than ten carats are rare.
  • Some deep, sky-blue, heat-treated stones are known to fade with prolonged exposure to sunlight.

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©1998
Updated 08/20/09
Comments and questions to helper@mail.utexas.edu
Department of Geological Sciences
The University of Texas at Austin