Fig. 77.—Tourmaline
Crystal.
To the crystallographer tourmaline is one of the most interesting of minerals. If the crystals, which are usually prismatic in form, are doubly terminated, the development is so obviously different at the two ends (Fig. 77) as to indicate that directional character in the molecular arrangement, termed the polarity, which is borne out by other physical properties. Tourmaline is remarkably dichroic, A brown stone, except in very thin sections, is practically opaque to the ordinary ray, and consequently a section cut parallel to the crystallographic axis, i.e. to the length of a crystal prismatically developed, transmits only the extraordinary ray. Such sections were in use for yielding plane-polarized light before Nicol devised the calcite prism known by his name (cf. [p. 44]). It is evident that tourmaline, unless very light in tint, must be cut with the table facet parallel to that axis, because otherwise the stone will appear dark and lifeless. The values of the extraordinary and ordinary refractive indices range between 1·614 and 1·638, and 1·633 and 1·669 respectively; the double refraction, therefore, is fairly large, amounting to 0·025, and, since the ordinary exceeds the extraordinary ray, its character is negative. The specific gravity varies from 3·0 to 3·2. The lower values in both characters correspond to the lighter coloured stones used in jewellery; the black stones, as might be expected from their relative richness in iron, are the densest. The hardness is only about the same as that of quartz, or perhaps a little greater, varying from 7 to 7½. It will be noticed that the range of refractivity overlaps that of topaz (q.v.) but the latter has a much smaller double refraction, and may thus be distinguished ([p. 29]). Unmounted stones are still more easily distinguished, because tourmaline floats in methylene iodide, while topaz sinks. The pyro-electric phenomenon (cf. [p. 82]) for which tourmaline is remarkable, although of little value as a test in the case of a cut stone, is of great scientific interest, because it is strong evidence of the peculiar crystalline symmetry pertaining to its molecular arrangement. Tourmalines range in price from 5s. to 20s. a carat according to their colour and quality, but exceptional stones may command a higher rate.
Tourmaline is usually found in the pegmatite dykes of granites, but it also occurs in schists and in crystalline limestones. Rubellite is generally associated with the lithia mica, lepidolite; the groups of delicate pink rubellite bespangling a background of greyish white lepidolite are among the most beautiful of museum specimens. Magnificent crystals of pink, blue, and green tourmaline have been found in the neighbourhood of Ekaterinburg, principally at Mursinka, in the Urals, Russia, and fine rubellite has come from the Urulga River, and other spots near Nertschinsk, Transbaikal, Asiatic Russia. Elba produces pink, yellowish, and green stones, frequently particoloured; sometimes the crystals are blackened at the top, and are then known locally as ‘nigger-heads.’ Ceylon supplies small yellow stones—the original tourmaline—which are confused with the zircon of a similar colour, and rubellite accompanies the ruby at Ava, Burma. Beautiful crystals, green and red, often diversely coloured, come from various parts, such as Minas Novas and Arassuahy, of the State of Minas Geraes, Brazil. Suitable gem material has been found in numerous parts of the United States. Paris and Hebron in Maine have produced gorgeous pink and green crystals, and Auburn in the same state has supplied deep-blue, green, and lilac stones. Fine crystals, mostly green, but also pink and particoloured, occur in an albite quarry near the Conn River at Haddam Neck, Connecticut. All former localities have, however, been surpassed by the extraordinary abundance of superb green, and especially pink, crystals at Pala and Mesa Grande in San Diego County, California. As elsewhere, many-hued stones are common. The latter locality supplies the more perfectly transparent crystals. Kunz states that two remarkable rubellite crystals were found there, one being 45 mm. in length and 42 mm. in diameter, and the other 56 mm. in length and 24 mm. in diameter. Madagascar, which has proved of recent years to be rich in gem-stones, supplies green, yellow, and red stones, both uniformly tinted and particoloured, which in beauty, though perhaps not in size, bear comparison with any found elsewhere.
CHAPTER XXV
PERIDOT
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 χρυσός, golden, and λίθος, 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 6½ 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 XXIX], 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 crystalline 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 refraction 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.
PLATE XXVII