Article: Let’s Talk Gemstones – Zircon Group

By Edna B. AnthonyMore from this author

Zircon is the single member of the zircon group of the nesosilicates that is suitable for use as a gemstone. In the nesosilicates, independent SiO4 tetrahedra are connected only by ionic bonds. Because the tetrahedra are not linked to form chains, sheets, groups, rings or a framework, an equidimensional crystal habit and the lack of distinct cleavage planes are prevalent.

Nesosilicate structures are generally determined by the size of the interstitial cations, the positively charged atoms occupying the spaces between the connected tetrahedra. Aluminum often replaces silicon in the silicates, but such substitution occurs less frequently in the nesosilicates. Dense atomic packing causes their relatively high specific gravity and hardness. Zircon exhibits these typical characteristics.

Hafnium, a metallic element, is always present in zircons, although the amount is usually less than four percent. Uranium and thorium are often present. The "self-irradiation", caused by the decay of these radio-active guests, results in damage to the atomic lattice and is responsible for the eventual alteration of the crystal to an isotropic glass. This process occurs over a long period of geologic time. Zircons thus affected are termed "metamict" and classified according to the extent of the damage sustained. "High" zircons have undergone little or no change to the lattice and are very acceptable for use as gems. "Low" and "intermediate" zircons have undergone more extensive destruction, but none present a hazard to humans.

Zircon is found worldwide in igneous rock formations and gem gravels. Thailand, Cambodia, and Sri Lanka, the source of the largest gem crystals and the greatest variety of colors, produce the major portion of the world's zircon gem material. The double-refractive uniaxial tetragonal crystals are often twinned and occur in tones of green, red, yellow, grey, orange, reddish-brown, and blue.

The refractive indices vary. Low zircon can be almost isotropic and exhibit indices of 1.78 - 1.85, with a birefringence of 0.0 to 0.008 and a density of 3.9 to 4.1. Intermediate material has indices of 1.85 - 1.93 for the ordinary ray and 1.84 - 1.97 for the extraordinary ray, birefringence of 0.008 to 0.043, and the density range of 4.1 to 4.65. High zircon refractive indices range from 1.92 - 1.94 (often 1.925) for the ordinary ray and an especially high 1.97 - 2.01 for the extraordinary ray. The birefringence is usually 0.059 but can be as low as 0.036. High zircon also has a greater specific gravity range of 4.65 - 4.8 but its usual density is 4.70.

The earliest use of zircon as a gemstone is unknown, but carved zircons have been found in some of the most ancient archaeological sites. Its name may have been derived from the Arabic word zarkun meaning "red" or a combination of the ancient Persian words zar (gold) and gun (color). Zircon has appeared in literature throughout history under a variety of names. Most of these designations are no longer in use, but some are still associated with specific colors. The beauty of zircon's sometimes adamantine luster and a dispersion approaching that of diamond is often not appreciated. The zircon cut, a variation of the brilliant cut that adds eight extra facets to the pavilion, was designed to take advantage of these properties. Heat treatment of colored material to enhance or change the color is an accepted practice. For decades, the process of repeatedly heating material in primitive ovens to obtain satisfactory results has also produced a more fragile and brittle product on which it is difficult to maintain a high polish. This and the use of the transparent grey tinged, colorless variety as a very inexpensive simulant for diamond has caused many persons to regard zircon with disdain. Its name has become synonymous with "cheap imitation." A small city in southern Sri Lanka supplied large quantities and gave its name to this variety often called the matara diamond.

Zircon Group Gemstone Properties

The especially descriptive name starlite for blue zircon is seldom used now, and the natural blue zircons that possess a unique blue color are rare. The zircon cut displays the properties of the blue gems to the greatest advantage. Blue crystals are found in Cambodia, Thailand, and Vietnam, but heat treatment of the red-brown material from this region produces most of the blue gems. The very characteristic absorption spectrum is nearly always present. The strength of the pale greenish-blue or yellowish-grey and the vivid blue pleochroism colors varies with the intensity of the color of the stone. Surprisingly, this gem has not been produced synthetically, but it is sometimes imitated by synthetic spinel.

Jacinth and hyacinth are terms that have been used extensively to denote red zircon. Most red, red-orange, or brick colored and violet-red gems have not undergone heat treatment. Crystals are found at Espaly, St. Marcel in France and in Sri Lanka, Thailand, and Cambodia. The oval and round mixed cuts with a brilliant-cut crown and a step-cut pavilion are the most common facet designs used for these gems, even though the zircon cut can be used to display its attributes to better advantage. With the exception of the spectrum, this red material possesses the typical zircon properties. The dichroscope reveals red and medium brown pleochroism colors. The orange gems from New South Wales, Australia display only a few lines of the distinctive zircon spectrum, and some red zircons may exhibit none.

In the past, yellow zircon was known by the name jargoon or jargon, a word we use today with a seemingly unrelated meaning. Yellow zircon includes the color range from pale to canary yellow, greenish yellow, and gold. It exhibits properties normal for zircon, but it can be confused with yellow sapphire. Sri Lanka is the main source of crystals, the heat-treated, red-brown material. A round or oval mixed cut is commonly used. A synthetic counterpart is not known to be produced, but it is imitated by man-made materials, such as YAG and cubic zirconia.

The readily available, richly-colored brown zircon makes a striking gem. Excellent cutting enhances its uncommon yellow-brown, reddish-brown, tobacco, and "black-tea" tones that are not found in other gemstones. Sri Lanka, Vietnam, Cambodia, and Burma furnish the majority of this material that also exhibits the normal properties consistent with the other varieties of zircon. Pleochroism colors of reddish-brown and yellowish-brown are seen with the dichroscope. No commercial imitation or production of brown synthetic zircon is known.

The distinctive colors of green zircon are frequently associated with the metamict state of the gemstones. Exceptionally transparent material in shades of brilliant green, yellow-green, and brownish-green is fairly common. Often, the material is cloudy. The usually obvious and strong birefringence common to most other zircons is rather difficult to detect in these gems at times. Despite the metamict state, the normal zircon spectrum is readily seen. Inclusions known as "spangles" are typical of these crystals as are "streaks" and angular zoning. Iron-stained interior tension fractures are common, and "silk" can be found less frequently. Burma and Sri Lanka are the main sources of green zircon.

Related Links:

Let's Talk Gemstones - Opal Part 1
Let's Talk Gemstones - Opal Part 2
Let's talk Gemstones - Staurolite
Let's Talk Gemstones - Peridot

Edna B. Anthony

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