Gems - Peridot
( Originally Published 1919 )
THE beautiful bottle-green stone, which from its delicate tint has earned from appreciative admirers the poetical sobriquet of the evening emerald, and which has during recent years crept into popular favour and now graces much of the more artistic jewellery, is named as a gem-stone peridot—a word long in use among French jewellers, the origin and meaning of which has been forgotten —but is known to science either as olivine, on account of the olive-green colour sometimes characterizing it, or as chrysolite. It is of interest to note that the last word, derived from xpvoos, golden, and XlOos, stone, was in use at the time of Pliny, but was employed for topaz and other yellow stones, while his topaz, curiously enough, designated the modern peridot (cf. p. 199), an inversion that has occurred in other words. The true olivine must not be confused with the jewellers' ` olivine,' which is a green garnet from the Ural Mountains (p. 217), Peridot is comparatively soft, the hardness varying from 61 to 7 on Mohs's scale, and is suitable only for articles which are not likely to be scratched ; the polish of a peridot worn in a ring would soon deteriorate. The choicest stones are in colour a lovely bottle-green (Plate XXIX, Fig. 2) of various depths ; the olive-green stones (Plate XX IX, Fig. 3) cannot compare with their sisters in attractiveness. The step form of cutting is considered the best for peridot, but it is sometimes cut round or oval in shape, with brilliant-cut fronts.
Peridot is a silicate of magnesium and iron, corresponding to the formula (Mg,Fe)2SiO4, ferrous iron, therefore, replacing magnesia. To the ferrous iron it is indebted for its colour, the pure magnesium silicate being almost colourless, and the olive tint arises from the oxidation of the iron. The latitude in the composition resulting from this replacement is evinced in the considerable range that has been observed in the physical characters, but the crystal-line symmetry persists unaltered ; the lower values correspond to the stones that are usually met with as gems. Peridot belongs to the orthorhombic system of crystalline symmetry, and the crystals, which display a large number of faces, are prismatic in form and generally somewhat flattened. The stones, however, that come into the market for cutting as gems are rarely unbroken. The dichroism is rather faint, one of the twin colours being slightly more yellowish than the other, but it is more pronounced in the olive-tinted stones. The values of the least and greatest of the principal indices of re-fraction vary greatly, from 1.650 and 1.683 to 1.668 and 1.701, but the double refraction, amounting to 0.033, remains unaffected. Peridot, though surpassed by sphene in extent of double refraction, easily excels all the ordinary gem-stones in this respect, and this character is readily recognizable in a cut stone by the apparent doubling of the opposite edges when viewed through the table facet (cf.p. 41). An equally large variation occurs in the specific gravity, namely, from 3'3 to 3.5.
Peridots of deep bottle-green hue command moderate prices at the present day, about 3s. a carat being asked for large stones ; the paler tinted stones run down to a few shillings a carat. The rate per carat may be very much larger for stones 0f exceptional size and quality.
Olivine, to use the ordinary mineralogical term, is a common and important constituent of certain kinds of igneous rocks, and it is also found in those strange bodies, meteorites, which come to us from outer cosmical space. Except in basaltic lavas, it occurs in grains and rarely in well-shaped crystals. Stones that are large and transparent enough for cutting purposes come almost entirely from the island Zebirget or St. John situated on the west coast of the Red Sea, opposite to the port of Berenice. This island belongs to the Khedive of Egypt, and is at present leased to a French syndicate. It is believed to be the same as the mysterious island which produced the ' topaz' of Pliny's time. Magnificent stones have been discovered here, rich green in colour, and 20 to 30, and occasionally as much as 80, carats in weight when cut; a rough mass attained to the large weight of 190 carats. Pretty, light-green stones are supplied by Queensland, and peridots of a less pleasing dark-yellowish shade of green, and without any sign of crystal form, have during recent years come from North America. Stones rather similar to those from Queensland have latterly been found in the Bernardino Valley in Upper Burma, not far from the ruby mines.