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.
- 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.
- 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
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.