GADOLINIUM GALLIUM GARNET
Galliate of gadolinium and gallium with a garnet-type structure It is an artificial product with no natural counter-part.
It is mainly produced by the Czochralsky method of crystallization from a melt, in cylindrical bars a few centimeters in diameter and up to 30 centimeters long, or in large, pedunculated pear shapes up to 10 centimeters in diameter. It is perfectly transparent and colorless or with a yellow tinge. In time, it may turn brownish in the light.
It has a hardness of 6.5, which is less than that of YAG and CZ or cubic zirconia. It has a very high density of 7.02 g/cm3, which is double that of diamond and also higher than that of the most recent imitations. The refractive index of 2.02 is above the range of ordinary refractor meters. The dispersion is 0.045, which is roughly the same as that of diamond. Production Because of certain special characteristics it displays, it was and still is mainly produced for use in various branches of electronics, especially in the United States.
Having very similar characteristics to diamond, in some respects more so than YAG, GGG partially replaced the latter as a diamond substitute in the 1970s, despite its much higher cost. But it has in turn been superseded by cubic zirconia, which has still better characteristics and is a lot cheaper.
Transparent and colorless (or with a yellow-brown tinge), it is seen almost exclusively in the form of brilliants. It bears a strong resemblance to diamond, but since the stone is not very hard, the facet edges often have small chips on them.
The refractive index is higher than that of YAG, a similar, more widely distributed product, but still less than that of diamond. Here, too, the effects on total internal reflection are such that if a stone is tilted gradually, a dark, transparent area is visible from the table facet, corresponding to certain pavilion facets, which let the light through instead of reflecting it. The girdle is usually different from that of diamond; less clearly cylindrical and with a striated appearance, as if it had been smoothed with a file. Its density is so high that, in the case of a loose stone 8 to 10 millimeters in diameter, it can be appreciated simply by weighing it in the hand.
Higher than that of modern synthetic stones, but less than most costly synthetics like emeralds. It is hardly ever seen on the market nowadays.