1911 Encyclopædia Britannica/Demantoid

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DEMANTOID, the name given by Nils Gustaf Nordenskiöld to a green garnet, found in the Urals and used as a gem stone. As it possesses high refractive and dispersive power, it presents when properly cut great brilliancy and “fire,” and the name has reference to its diamond-like appearance. It is sometimes known as “Uralian emerald,” a rather unfortunate name inasmuch as true emerald is found in the Urals, whilst it not infrequently passes in trade as olivine. Demantoid is regarded as a lime-iron garnet, coloured probably by a small proportion of chromium. The colour varies in different specimens from a vivid green to a dull yellowish-green, or even to a brown. The specific gravity of an emerald-green demantoid was found to be 3.849, and that of a greenish-yellow specimen 3.854 (A. H. Church). The hardness is only 6.5, or lower even than that of quartz—a character rather adverse to the use of demantoid as a gem. This mineral was originally discovered as pebbles in the gold-washings at Nizhne Tagilsk in the Ural Mountains, and was afterwards found in the stream called Bobrovka, in the Sysertsk district on the western slope of the Urals. It occurs not only as pebbles but in the form of granular nodules in a serpentine rock, and occasionally, though very rarely, shows traces of crystal faces.  (F. W. R.*)