PERIDOT, sometimes written peridote, a name applied by jewelers to “noble olivine,” or that kind of olivine which can be used as a gem-stone (see Olivine). The word peridot is an old trade-term, of unknown origin, used by French jewelers and introduced into science by J. R. Haüy. Peridot is practically the same stone as chrysolite (q.v.), though it is convenient to restrict that term to transparent olivine of pale yellowish green colour, and to apply the term peridot to those kinds which are darker and decidedly green: the colour, which is due to the presence of ferrous iron, is never vivid, like that of emerald, but is usually some shade of olive-, pistachio- or leek-green. Although the stone is sometimes cut en cabochon, and in rose-form, the cutting best adapted to display the colour is that of a table or a step-cut stone. Unfortunately the hardness of peridot is only about 6·5, or but little above that of glass, so that the polished stone readily suffers abrasion by wear. In polishing peridot the final touch is given on a copper wheel moistened with sulphuric acid.
Although olivine has a fairly wide distribution in nature, the varieties used as gem-stones are of very limited occurrence. Much mystery for a long time surrounded the locality which yields most of the peridot of commerce but it is now identified with the island of St John, or Isle Zeboiget, in the Red Sea, where it occurs, as shown by M. J. Couyat. in an altered dunite, or olivine rock (Bull soc. franç. min., 1908). This is probably the Topaz Isle, τοπάζιος νῆσος, of the ancients. It is generally held that the mineral now called topaz was unknown to ancient and medieval writers, and that their τοπάζιον was our peridot. Such was probably the Hebrew pitdah, translated topaz in the Old Testament. Dr G. F. Kunz has suggested that the peridots of modern trade are largely derived from old jewelry. The famous shrine of the Three Kings in Cologne Cathedral contains a large peridot, which has commonly been regarded as an emerald. It is notable that pebbles of transparent olivine, fit for cutting, are found in the United States in Montana, Arizona and New Mexico; in consequence of their shape and curiously pitted surface they are known as “ Job's tears.” (F. W. R.*)