Zircon October and December’s Birthstone
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
HEAT SENSITIVE No
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.”
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.
A natural gemstone, which occurs in all shades of the color spectrum; zircon is found in many Asian countries, notably Sri Lanka as well as in Brazil, Australia and East Africa. Colorless when pure, the zirconium silicate takes on various shades due to impurities.
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.
Text and photos courtesy of Barbara Smigel at Artistic Colored Stones.