Jewelry Lovers

Nature has conferred on zircon several quite outstanding optical properties which furnish brilliant-cut specimens with a particularly striking appearance. High adamantine luster and very vivid fire elevate zircon into the immediate proximity of diamond. Its specific gravity is unequalled by any other transparent gemstone. Its high refractive index of 1.95 and powerful color-dispersive ability of 0.038 make it an impressive gemstone, whose fascination is further increased by the remarkable fact that these values are by no means constant, being subject to a certain variability about whose ultimate rules there is still no general understanding. The visible evidence of double refraction so typically distinct in zircon will not escape the notice of the alert observer. It manifests itself in the lack of sharpness of its pavilion edges when the eye looks from the sharp crown edges into the depth of the stone. Many other gemstones also possess this property of splitting the incident light into two different strongly refracted rays, so that everything seen right through these stones appears doubled.

The colors of zircon offer a profusion of scintillating beauty and complement the optical merits in a highly effective way. The pure substance is in itself colorless, as neither the zirconia nor the silica in its chemical composition are in the least color-bearing. It is extremely probable that the zircon colors are not exclusively due to the chromoforous elements present, but also correlate with the content of the radioactive elements uranium and thorium which are included in varying amounts as trace elements. It is the presence of these radioactive atoms which enables the colors of zircon to be altered by heating, whereby the secret of its manifold shades of color is also revealed.

The most coveted colors—blue, extremely rare occurrence in nature, and colorless, the gemstone most closely resembling diamond—are such heat-induced varieties: the blue is produced by heating zircon in a vacuum, while firing in a stream of oxygen decolors zircon. The blue zircon enjoyed great favor during the first half of this century; on the other hand, because of its high refraction and strong fire, the colorless variety is often used as a diamond substitute in imitation jewelry. Unfortunately not all artificially colored zircons prove to be color-fast, for under ultraviolet rays or in daylight they may revert to their original colors.

Zircon, which crystallizes in a tetragonal (four-sided) form, has taken part in different gemstone evolutions and at times accompanies acid plutonic rocks as an accessory component, and at times belongs to pegmatites (vein rocks) and to some crystalline schists.

It is thus found in all kinds of rocks and deposits throughout the world; but the richest occurrences of gem zircon are found in Cambodia, Ceylon, Burma, Thailand, and Vietnam, and are without exception alluvial placer deposits, from which the zircon is washed out as a companion mineral of ruby, sapphire, and many other gemstones. The zircons of Ceylon are outstanding for their particularly gorgeous color range, since the island, an extraordinarily fecund source of valuable stones, brings to light the whole broad spectrum of hues, except blue. Burma, in addition to the rare blue, plucks mainly cognac brown, green, and red varieties from the dazzling bouquet of colors, while the Indochinese deposits are characterized by yellow, brown, and reddish-brown specimens.

It was zircon which first enabled the age of a gemstone deposit to be determined. In many zircons, which have a very high thorium or uranium content, the crystalline atomic structure gradually collapses under the radioactive bombardment into the amorphous state of an irregular aggregate of zirconia and silica. The zircons then become cloudy and simultaneously take on a green coloration; the physical values drop; the strong double refraction disappears, and a part of the uranium and thorium alters to lead. The quantitative ratio between uranium and thorium on the one hand, and lead on the other, which is today measurable, permits determination of the age of the zircon and hence of its deposit. In this way it has been calculated, for example, that the gem gravels of Ceylon are about 5 5o million years old. A few years ago the discovery of ekanite, a new radioactive gemstone from Ceylon, enabled the author to confirm these age determinations.

Zircon, with its many features, was in earlier times called “hyacinth.” The name is supposedly to be traced back, through intervening oriental stages, to the Greek hyakinthos. The more recent name zircon comes from the Persian word zargun, which means gold-colored.

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