Zircon is an underrated but magnificent gemstone that has been worn and treasured since ancient times. Don't be confused by the name. Zircon is not the synthetic material known as cubic zirconia. It's a completely distinct, natural species. Coincidentally, zircon is the original diamond simulant. When properly cut, colorless stones can make beautiful and convincing diamond substitutes and even outshine mediocre diamonds. Its high dispersion is close to that of diamond. However, the simulation only goes so far. Unlike diamond, zircon is brittle, and its facet edges tend to chip and wear. Zircon must be worn carefully to prevent damage. As a ring stone, it should have a protective setting. If worn occasionally, it can be used in any type of jewelry. Zircons are stunning gemstones in their own right. They are found in a wide range of colors, and many additional colors can be produced by heat treatments.
The International Gem Society (IGS) has a list of businesses offering gemstone appraisal services.
|Crystallography||Tetragonal. Crystals prismatic, pyramidal; often twinned; rounded pebbles.|
|Refractive Index||Low, 1.78 - 1.85. High 1.92 - 2.01|
|Colors||Reddish brown, yellow, gray, green, red; various other colors induced by heating.|
|Luster||Vitreous to adamantine; sometimes greasy.|
|Polish Luster||Vitreous to adamantine|
|Fracture Luster||Vitreous to subadamantine|
|Hardness||Low 6 to 7.5 high|
|Specific Gravity||Low 3.9 - 4.1. High 4.65 - 4.80|
|Cleavage||Imperfect. Very brittle.|
|Dispersion||0.039 for all zircon types.|
|Luminescence||See "Identifying Characteristics."|
|Spectral||See "Identifying Characteristics."|
|Enhancements||Virtually all blue zircon is heat treated.|
|Special Care Instructions||Facet edges wear off. Use protective settings for ring use.|
|Transparency||Transparent to opaque|
|Absorption Spectrum||See "Identifying Characteristics."|
|Formula||ZrSiO4 + Fe, U, Th, Hf|
|Pleochroism||Distinct in blue stones: deep sky blue/colorless to yellowish gray. Red: red/ clove brown. Brown: reddish brown/yellowish brown.|
|Optics||Uniaxial (+). See "Varieties" for more information.|
|Etymology||From the Arabic zargun, from the Persian zar for “gold” plus gun for “color.” The name is ancient.|
|Occurrence||In igneous rocks worldwide, especially granites. Also found as alluvial material.|
|Inclusions||Angular zoning and streaks are sometimes seen in the low type. Some silk is seen occasionally, as well as tension cracks and epigenetic cracks stained with iron oxides. Metamict pieces may have bright fissures known as angles.|
As a species, zircon has many interesting characteristics. Some, such as its high refractive index (RI) and dispersion, seem to have made it the “natural choice” for a diamond simulant, so to speak. Interestingly, despite its long history as a diamond substitute, the modern December birthstone is actually less common than diamond. Zircon’s birefringence is so strong that gem cutters must orient the table of the stone to the optic axis. Otherwise, the interior of the stone may look fuzzy due to facet image doubling.
Some zircon crystals pick up small amounts of radioactive uranium and thorium during growth. This radiation is barely measurable. However, over millions of years, the radiation breaks down the crystal structure. These stones, usually green, become metamict. That is, they are materials that have lost their crystalline structure and become amorphous due to radiation. Metamict zircons have a lower RI and brilliance than the crystalline type.
Zircon is classed as high, medium, or low based on its level of deterioration. (These are also called alpha, beta, and gamma.) The classes are easy to distinguish because the properties change in an even progression.
High zircon is fully crystalline and has the highest properties. Material slightly damaged by radiation is called intermediate zircon. The low type is metamict. Interestingly, dispersion is the same for both high and low varieties while other optical properties vary. Low zircon usually has a cloudy texture.
|Low Zircon||Intermediate||High Zircon|
|Colors||Green, brown, orange||Brownish green, dark red||Colorless, blue, brownish orange|
|RI||o = 1.78 - 1.815 (almost isotropic)||o = 1.83 – 1.93
e = 1.84 – 1.970
|o = 1.92 – 1.94 (often 1.925)
e = 1.97 – 2.01 (often 1.984)
|Birefringence||0 – 0.008||0.008 – 0.043||0.036 – 0.059 (usually 0.059)|
|Density||3.95 – 4.20 (usually about 4.0)||4.08 – 4.60||4.6 – 4.80 (usually 4.70)|
The most obvious way to distinguish a zircon presented as a substitute diamond from an actual diamond is by the former’s birefringence.
All classes of faceted zircon can be identified by abraded or worn facet edges.
The fluorescence of zircon is variable. Some material is inert. Other crystals glow intensely. Mustard yellow and yellow-orange are typical fluorescent colors (SW). Some zircons glow dull yellow in LW and may phosphoresce. Zircon may be whitish, yellow, greenish, or violet-blue under X-rays.
- Red to orange red: inert to strong, yellow to orange (SW).
- Yellow to orangish yellow: inert to moderate yellow to orange (LW and SW).
- Green: usually inert.
- Blue: inert to moderate, light blue (LW).
- Brown: inert to very weak red (SW).
Most zircons show a strong absorption pattern that’s very useful for identification. Green stones from Myanmar may show more than forty lines, while orange gems from New South Wales, Australia may show only a few lines. Low zircon and heat-treated stones have a weaker display.
- Most zircons show a strong line at 6535 even in types where a strong spectrum is otherwise absent. They may also have lines at 6910, 6830, 6625, 6605, 6210, 6150, 5895, 5625, 5375, 5160, 4840, 4600, and 4327.
- Heat-treated stones (colorless, blue, and golden brown) may show only one fine line at 6535 and also a weaker line at 6590.
- Red and brown zircons may display no spectrum at all.
Almost all colorless and blue zircons have been heat treated. This treatment is undetectable and stable. The popular blue, colorless, and golden yellow shades can only be produced by heating. The stones that yield these lovely colors are usually reddish-brown. This treatment is less common with other colors.
- Heating helps to crystallize partially metamict zircons. This raises specific gravity and sharpens the absorption spectrum.
- Heating green Sri Lankan zircon makes it paler in color. Red-brown Sri Lankan material becomes colorless, sometimes reddish-violet.
- Heating red-brown Thai and Cambodian stones turns them colorless, blue, or golden.
- Brownish stones are often heated either with or without oxygen present to achieve shades of blue and golden yellow.
- Rich, slightly greenish blue, heated zircons were once marketed as “Starlite.” The term never caught on.
The primary sources of zircon are Sri Lanka and Southeast Asia, but it has also been found in Mexico and other localities.
- United States: South Dakota; Colorado; Oklahoma; Texas; Maine; Massachusetts; New York; New Jersey.
- Sri Lanka: one of the most important zircon areas, material of all colors, in gravels. Cat’s eye also found.
- Myanmar: yellowish and greenish stones found in gem gravels with ruby. Complex absorption spectrum in these stones.
- Thailand: one of the most important commercial sources of gem-grade zircon.
- Cambodia: chief source of material that heat treats colorless and blue.
- France: red crystals at Espaly, St. Marcel.
- Quebec and Ontario, Canada: dark, opaque crystals up to 15 pounds, yield only tiny gems.
- Arendal, Norway; New South Wales, Australia: fine gem material (orange).
- Emali, Tanzania: white zircon pebbles.
- Russia; Korea; Germany; Brazil; Mexico; Pakistan.
The largest zircon gems are from Southeast Asian gem gravels.
- Smithsonian Institution (Washington, D.C.): 118.1 (brown, Sri Lanka); 97.6 (yellow-brown, Sri Lanka); 75.8 (red-brown, Myanmar); 64.2 (brown, Thailand); 23.5 (green, Sri Lanka); 23.9 (colorless, Sri Lanka); 103.2 (blue, Thailand).
- Geology Museum, London: 44.27 (blue); 22.67 (golden); 14.34 (red; 21.32 (white).
- Royal Ontario Museum (Toronto, Ontario, Canada): 23.8 (brown); 17.80 (blue); 61.69 (blue, step-cut).
- American Museum of Natural History (New York): 208 (greenish blue, Sri Lanka).
- Hyacinth or jacinth: transparent reddish brown. (Note: this term is also applied to hessonite, a variety of garnet).
- Jargoon or jargon: light yellow to colorless.
- Beccarite: green
- Melichrysos: yellow
- Sparklite: colorless
- Starlite and Stremlite: blue
Colorless zircons have been sold as “Matara diamonds” and “Ceylon diamonds.” Although zircons are used to simulate diamonds, selling them as actual diamonds is unethical.
Blue zircons have been deceptively sold as “Siam aquamarines.”
Consult our List of False or Misleading Gemstone Names for more examples of deceptively labeled gems.
Consult our Gemstone Care Guide for recommended cleaning methods.
by Dr. Joel Arem; Donald Clark, CSM IMG; Barbara Smigel, PhD GG