DIAMOND

Diamond Substitutes. In the past, various transparent white gems have been used for diamonds. None has the diffusion or fire of a real diamond, and such substitutes can easily be detected today. Over the years, the value put one diamonds has meant that a Substitute has been sought. General Electric has produced a diamond in the laboratory with the same hardness and dispersion as a natural diamond. It is, in fact, a real diamond of pure carbon, but the cost of making it is so high that it has no advantage over natural diamonds.

Other synthetics have not been as successful. Two of these are Yttrium Aluminum Garnet (YAG) and Gadolinium Gallium Garnet (GGG). While YAG has a hardness of 8, it is low in dispersion with none of the fire of a diamond. GGG is as hard as YAG and has about the same dispersion as a diamond, but it tends to turn brownish with wear. Another synthetic is synthetic rutile, called Titania.

Although natural rutile is opaque and red, synthetic rutile can be made in any color, including the transparent white of a diamond. The problem is that it is relatively soft and shows too much dispersion. More important than any of these is a synthetic stone called Cubic Zirconium or Zirconia developed by the Russians in the late 1960s and now being manufactured in the United States under both of those names as well as trade names such as Diamonair II. With a hardness of 7'/z to 8'/: on the Mohs scale and a refractive index and dispersion that balance out to come startlingly close to a genuine diamond, it can fool even an expert without a magnifying lens.

Diamond doublets are another substitute. The top is genuine diamond, and the bottom is another material.

FELDSPAR

Feldspar is one of the most common minerals on earth. With a hardness of 6 to 6'/2 on the Mohs scale, it is relatively soft for gem use, although several types of feldspar are used in jewelry. It has good cleavage properties.Moonstone. Moonstone is the best-known feldspar. Nearly opaque moonstones may be pink, green, yellow, beige, brown, gray, or ,white.