Gems - Tourmaline - Rubellite
( Originally Published 1919 )
TOURMALINE is unsurpassed even by corundum in variety of hue, and it has during recent years rapidly advanced in public favour, mainly owing to the prodigal profusion in which nature has formed it in that favoured State, California, the garden of the west. Its comparative softness militates against its use in rings, but its gorgeous coloration renders it admirably fitted for service in any article of jewellery, such as a brooch or a pendant, in which a large central stone is required. Like all coloured stones it is generally brilliant-cut in front and step-cut at the back, but occasionally it is sufficiently fibrous in structure to display, when cut en cabochon, pronounced chatoyancy.
The composition of this complex species has long been a vexed question among mineralogists, but considerable light was recently thrown on the subject by Schaller, who showed that all varieties of tourmaline may be referred to a formula of the type 2S 1 02. 3 B203.(9 —x)[(A1,Fe)203].3x[(Fe,Mn,Ca, Mg,K2,Na2,Li2,H2)O].3H20. The ratios of boric oxide, silica, and water are nearly constant in all analyses, but great variation is possible in the proportions of the other constituents. Having regard to this complexity, it is not surprising to find that the range in colour is so great. Colourless stones, to which the name achroite is sometimes given, were at one time exceedingly rare, but they are now found in greater number in California. Stones which are most suited to jewellery purposes are comparatively free from iron, and apparently owe their wonderful tints to the alkaline earths ; lithia, for instance, is responsible for the beautiful tint of the highly prized rubellite, and magnesia, no doubt, for the colour of the brown stones of various tints. Tourmaline rich in iron is black and almost opaque. It is a striking peculiarity of the species that the crystals are rarely uniform in colour throughout, the boundaries between the differently coloured portions being sharp and abrupt, and the tints remarkably in contrast. Sometimes the sections are separated by planes at right angles to the length of the crystal, and sometimes they are zonal, bounded by cylindrical surfaces running parallel to the same length. In the latter case a section perpendicular to the length shows zones of at least three contrasting tints. In the Brazilian stones the core is generally red, bounded by white, with green on the exterior, while the reverse is the case in the Californian stones, the core being green or yellow, bounded by white, with red on the exterior. Tourmaline may, indeed, be found of almost every imaginable tint, except, perhaps, the emerald green and the royal sapphire-blue. The principal varieties are rose-red and pink (rubellite) (Plate XXVII, Fig. I), green (Brazilian emerald), indigo-blue (indicolite), blue (Brazilian sapphire), yellowish green (Brazilian peridot) (Plate XXVII, Fig. 2), honey-yellow (Ceylonese peridot), violet-red (siberite), and brown (Plate XXVII, Fig. 8). The black, opaque stones are termed schorl.
The name of the species is derived from the Ceylonese word, turamali, and was first employed when a parcel of gem-stones was brought to Amsterdam from Ceylon in 1703 ; in Ceylon, however, the term is applied by native jewellers to the yellow zircon commonly found in the island. Schorl, the derivation of which is unknown, is the ancient name for the species, and is still used in that sense by miners, but it has been restricted by science to the black variety. The ` Brazilian emerald' was introduced into Europe in the seventeenth century and was not favourably received, possibly because the stones were too dark in colour and were not properly cut; that they should have been confused with the true emerald is eloquent testimony to the extreme ignorance of the characters of gem-stones prevalent in those dark ages. Achroite comes from the Greek, axpoos, without colour.
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 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 7 1/2. 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 pyroelectric 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, extraordinary and ordinary refractive indices rangebetween 1.614 and 1.638, and 1.633 and 1669 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 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 ; some-times 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 Arassuhy, 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 trans-parent 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.