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 12SiO_{2}.3B_{2}O_{3}.(9-_x_)[(Al,Fe)_{2}O_{3}].3_x_[(Fe,Mn,Ca, Mg,K_{2},Na_{2},Li_{2},H_{2})O].3H_{2}O. 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. 1), green (Brazilian emerald), indigo-blue (indicolite), blue (Brazilian sapphire), yellowish green (Brazilian peridot) (Plate XXVII, Fig. 2),