Zircon October and December’s Birthstone
Formula: ZrSiO4 + Fe, U, Th, Hf.
Crystallography: Tetragonal. Crystals prismatic, pyramidal; often twinned; rounded pebbles.
Colors: Reddish brown, yellow, gray, green, red; various other colors induced by heating.
Luster: Vitreous to adamantine; sometimes greasy.
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.
Birefringence: 0.008-0.069. Optically uniaxial (+).
Dispersion: 0.039 for all zircon types.
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.
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 crystals may have bright fissures known as angles.
Heating Effects: 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.
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.
Refractive Index Low, 1.78 – 1.85. High 1.92 – 2.01
Hardness Low 6 to 7.5 high
Specific Gravity Low 3.9 – 4.1. High 4.65 – 4.80
Special Care Instructions Facet edges wear off, caution if putting in a ring.
Enhancements Virtually all blue zircon is heat treated.
*Wearability is graded as Excellent, Very Good, Good, Poor, and Forget It! For more details see the article on “Hardness and Wearability.”
Pleochroism: Distinct in blue stones: deep sky blue/colorless to yellowish gray. Red: red/ clove brown. Brown: reddish brown/yellowish brown.
Occurrence: 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.
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).
Comments: 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.
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.
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.
Name: Zircon is from the Arabic zargun, from the Persian zar (gold) plus gun (color). The name is ancient.
The IGS gratefully acknowledges Dr. Joel Arem and Barbara Smigel of Artisitc Colored Stones for generously allowing us to utilize their content; both photos and text above with attribution.