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, ἄχροος, without colour. [Illustration: 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