Gem/Gem Topic of the Month

Each month this section will feature either a topic of interest to gem lovers or one special gemstone with background on the material and its value.

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January, 1999 (Revised, August, 2004)

Zircon

The beautiful, historically important gemstone, zircon, has unfortunately, in recent years been tarnished by its name-only similarity to cheap, ubiquitous, synthetic cubic zirconia. The two are totally distinct in their chemistry, optical properties and origins, though.

A natural gemstone, which occurs in all shades of the color spectrum; zircon is found in many Asian countries, notably Cambodia and 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 most common color in the natural rough is yellowish brown.

Colorless zircons were the original diamond simulants, and, if well cut, can be visually 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 a colorless state or 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 zircon crystals contain naturally radioactive thorium and uranium. Over time, this radioactivity breaks down the crystal structure so that such stones (usually green) tend to an amorphous, glass-like state, with a lower refractive index and luster than the crystalline type. These gems are referred to as "metamict" or "low zircon" and are sought as collector's items.

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. With a luster just short of diamond and its high refractive index, exceptional brilliance is possible when a piece is well cut and polished. Stones with light body color may show strong dispersion which is a bonus. Collectors appreciate the many color forms but especially seek out reds and greens.

 

[Metamict Zircon] [Cat'seye Zircon]

Value

Medium dark, pure blue stones have the highest value, followed closely by those with a saturated greenish blue color. Red stones (always with some orangey hue) in larger sizes also command relatively high prices. Cut is an important factor in value as the vast majority of zircons on the market, especially blues, have been native cut. A custom cut, therefore, enhances value. Sizes of zircon rough is not as limited as in some species, so although there is some premium per carat for larger stones it is not exponential. Cat'seye stones are quite rare in this species and especially valued.


Gemological Data

Makeup: ZrSiO4

Luster: resinous to adamantine

Hardness: 7.5

Crystal structure: tetragonal

Fracture: concoidal

Cleavage: none

Density: 4.69

RI: 1.93 - 1.98

Birefringence: .059

Dispersion: .038

All text and images, unless otherwise designated, © 2004 Barbara Smigel

 
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Go to: Homepage -- what's new in faceted gems -- what's new in designer cabochons and gem carvings -- gem of the month -- gem of the month archive -- birthstone of the month -- key to all the codes used on the ACS site -- definitions of terms used on the ACS site -- how to order -- about ACS -- about the ACS cutters -- settings for these gems --faceting information -- purchase UltraTec equipment