Zircon ValueBlue and colorless, very common. Other colors somewhat common. Blue: These are very strongly greenish blue to slightly greenish blue. Highest values as you get closer to pure blue. High saturation and tone increase value. (Tones 2-6, saturation 1-4.) Medium dark, pure blue stones have the highest per carat value, estimated by Sinkankas (Standard Catalog of Gem Values, 2nd. Ed), at $150-$300 in large sizes, followed by blue with a slight greenish cast at about $100/ct. Red stones (always with some orangey hue) in larger sizes may command $100/ ct as well.
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|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|
|Birefringence||0.008-0.069. Optically uniaxial (+).|
|Cleavage||Imperfect. Fracture conchoidal. Very brittle. Zircon crystals usually contain traces of radioactive elements such as U and Th. These decay within the crystals, and over a period of thousands of years result in severe damage to the crystal structure of the host zircon. The damage can be severe enough to destroy the lattice itself, ultimately decomposing the zircon internally into a mixture of quartz and zirconium oxide that is essentially amorphous. This damaged, nearly isotropic zircon is called low zircon, whereas the undecayed material is called high zircon. Material slightly damaged by radiation is called intermediate zircon, and a complete transition exists between the low and high type.|
|Dispersion||0.039 for all zircon types.|
|Stone Sizes||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, Burma); 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).|
|Luminescence||The fluorescence of zircon is variable. Some material is inert, other crystals glow intensely. Mustard yellow is a typical fluorescent color (SW), also yellow-orange. Some zircons glow dull yellow in LW and may phosphoresce. Zircon may be whitish, yellow, greenish, or violet-blue in X—rays.|
|Spectral||Zircon spectra are very distinctive and useful in identification. The strongest pervasive line is at 6535, seen even in types where a strong spectrum is absent. There are many narrow lines and strong bands across the whole spectrum, ranging from more than 40 lines (Burma green stones) to only a few lines (orange gems from New South Wales, Australia). Heat-treated stones and low types have a weak spectrum. Colorless, blue, and golden-brown (all heat-treated) stones display one fine line atv6535, and perhaps also a line at 6590. The complex spectrum of other zircons includes lines at 6535, 6910, 6830, 6625, 6605, 6210. 6150, 5895, 5625, 5375. 5160, 4840, 4600, and 4327. Red zircons may display no spectrum at all.|
|Enhancements||Virtually all blue zircon is heat treated.|
|Special Care Instructions||Facet edges wear off, caution if putting in a ring.|
|Transparency||Transparent to Opaque|
|UV Long||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).|
|Identifying Characteristics||All zircon: Abraded facet edges. High Zircon: Silk occasionally, tension cracks sometimes stained with iron oxide.|
|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.|
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 crystals may have bright fissures known as angles.
Heating helps to crystallize partially metamicted zircons and results in a higher specific gravity; the absorption spectrum also sharpens. Heating green Sri Lankan zircon makes it paler in color. Red-brown Sri Lankan material becomes colorless, sometimes reddish-violet. Red—brown Thai and Cambodian stones turn colorless, blue, or golden.
Heating: Heat treating helps to crystallize partially metamicited zircon. This raises the specific gravity and sharpens the spectrum. Sri Lankan green material becomes lighter, and red from Sri Lanka becomes colorless or brownish red. Reddish brown material, primarily from Thailand and Cambodia, turns blue, colorless, or gold. Almost all colorless and blue zircons have been heat treated; it is less common with other colors. Undetectable, stable.
Most zircons show a strong absorption pattern and with as many as forty lines! Low zircon and heat treated stones have a weaker display and only a few red and brown stones may show no spectrum
Most zircons show a strong line at 653.5 nm. They may also have lines at 65.35, 691.0, 683.0, 662.5, 660.5, 621.0, 615.0, 589.5, 562.5, 537.5, 516.0, 484.0, 460.0, and 432.7.
Heat treated colorless, blue and gold may show only one fine line at 653.5, may also show a weaker line at 659.0.
In igneous rocks worldwide, especially granites. Also found as alluvial material.
- South Dakota; Colorado; Oklahoma; Texas; Maine; Massachusetts; New York; New Jersey.
- USSR; Korea; Germany; Brazil.
- Sri Lanka: one of the most important zircon areas, material of all colors, in gravels.
- Burma: 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 zircon.
- Cambodia: chief source (although no current production) 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.
Comments by Dr. Joel Arem
Zircon is an underrated but magnificent gemstone. When it is properly cut, it rivals diamond in beauty, but often the cutting is not correct and the gem is relatively dull and lifeless. The dispersion is very high, close to that of diamond. Zircon is very brittle and edges of stones are easily chipped and abraded. Zircon must be worn carefully to prevent damage. The range of color in the material is wide, and many additional colors are produced by heating.
High Zircon is fully crystalline and has the highest properties, whereas the low type is metamic, due to bombardment of the internal crystal structure by alpha particles released by U and Th. In cases of extreme damage to the structure, the material may appear isotropic, with lower refractive indices and less brilliance when cut. Interestingly, the dispersion is the same for both the high and low types. The popular blue color can be produced only by heating zircon; the same is true for the colorless and golden yellow shades. The crystals that yield these lovely colors are usually reddish-brown. Large, fine-colored zircons are very rare stones, and even smaller fine ones are seldom seen in jewelry today. Catseye zircon has also been reported from Sri Lanka.
Comments by By Dr. Barbara Smigel
The beautiful, historically important gemstone, zircon, has unfortunately in recent years become tarnished by its name-only similarity to cheap, ubiquitous, synthetic cubic zirconia. Of course the two are totally distinct in their chemistry, optical properties and origins.
The original diamond substitute, colorless stones, if well cut, can be convincing, but are easily distinguished from diamond by their double refraction and the tendency to wear along facet edges. Brownish stones are often heated either with or without oxygen present to achieve shades of blue and golden yellow. The rich, slightly greenish blue heated zircons had at one time been marketed as “starlite”, but the term never caught on.
Some crystals contain radioactive thorium and uranium. Over time, the radioactivity breaks down the crystal structure so that such stones (usually green) tend to an amorphous structure, with a lower refractive index and luster than the crystalline type. The high birefringence of zircon makes it necessary for the cutter to orient the table of the stone to the optic axis; otherwise the interior may look fuzzy, due to facet image doubling.
Round stones are often given a “zircon” cut which is similar to a standard round brilliant cut with an extra tier of facets at the culet. Although use in rings should be limited to protective settings or occasional wear jewelry, in general zircon is a magnificent jewelry stone. Collectors appreciate the many color forms but especially seek out reds and greens.
Comments by Donald Clark, CSM IMG
Zircon is one of our most beautiful gems and has been used since antiquity. Its primary sources are Sri Lanka and Southeast Asia, but it has also been found in Mexico and other localities.
As a species, zircon has many interesting characteristics. Its high RI and dispersion rival diamond. In fact, a well cut colorless zircon can outshine a mediocre diamond. Unfortunately, they are not nearly as common as diamonds and they are brittle. One of zircons identifying characteristics is abraded facet edges. Zircons do not wear well as ring stones.
Zircon crystals pick up small amounts of uranium and thorium during growth. Though the radiation is barely measurable, over millions of year the crystal structure slowly deteriorates. Given enough time, they become metamict, or amorphous.
Zircon is classed as high, medium, or low based on their level of deterioration. (These are also called alpha, beta and gamma.) They are easy to distinguish because the properties change in an even progression.
|Low Zircon||Intermediate||High Zircon|
|Colors||Green, brown, orange||Brownish green, dark red||Colorless, blue, brownish orange|
|RI||1.78 1.85||1.85 1.93||1.92 1.94|
|Birefringence||0 – .008||.008 – .043||.036 – .059|
|Density||3.9 4.0||4.1 4.65||4.65 4.80|
It is interesting to note that while the other optical properties vary, the dispersion remains the same for all three types.
This is primarily for your information. For most purposes, it is enough to identify the stone simply as zircon, described by the color. (I.E. Blue zircon.)
Zircon is from the Arabic zargun, from the Persian zar (gold) plus gun (color). The name is ancient.
Variety and Trade Names
- High and medium zircon: transparent, colorless, blue, yellow, yellow green, brownish green, brown, (sometimes with red flashes,) orangish red.
- Low zircon: Transparent, brownish to yellowish green, occasionally deep brown or orange. Usually has a cloudy texture.
- Hyacinth or jacinth: transparent reddish brown. (Note that this term is also applied to hessonite garnet.)
- Jargoon or Jargon: light yellow to colorless.
- Beccarite: green
- Melichrysos: yellow
- Sparklite: colorless
- Starlite and stremlite: blue
- Matara diamond: colorless
- Ceylon diamond: colorless
- Siam Aquamarine: blue zircon
Source/Attribution: Barbara Smigel of Artistic Colored Stones, Dr. Joel Arem, Donald Clark, CSM IMG