Topaz is a silicate of aluminium corresponding to the formula [Al(F,OH)]_{2}SiO_{4}, which was established in 1894 by Penfield and Minor as the result of careful research. Contrary to the general idea, topaz is usually colourless or very pale in tint. Yellow hues of different degrees, from pale to a rich sherry tint (Plate I, Fig. 9), are common, and pure pale blue (Plate I, Fig. 7) and pale green stones, which often pass as aquamarine, are far from rare. Natural, red and pink, stones are very seldom to be met with. It is, however, a peculiarity of the brownish-yellow stones from Brazil that the colour is altered by heating to a lovely rose-pink. Curiously, the tint is not apparent when the stone is hot, but develops as it cools to a normal temperature; the colour seems to be permanent. Such stones are common in modern jewellery. Although the change in colour is accompanied by some slight rearrangement of the constituent molecules, since such stones are invariably characterized by high refraction and pronounced dichroism, the crystalline symmetry, however, remaining unaltered, the cause must be attributed to some change in the tinctorial agent, probably oxidation. The yellow stones from Ceylon, if treated in a similar manner, lose their colour entirely. The pale yellow-brown stones from Russia fade on prolonged exposure to strong sunlight, for which reason the superb suite of crystals from the Urulga River, which came with the Koksharov collection to the British Museum, are kept under cover. The name of the species is derived from _topazion_ (τοπάζειν, to seek), the name given to an island in the Red Sea, which in olden times was with difficulty located, but it was applied by Pliny and his contemporaries to the yellowish peridot found there. The term was applied in the Middle Ages loosely to any yellow stone, and was gradually applied more particularly to the stone that was then more prevalent, the topaz of modern science. As has already been pointed out (p. 111), the term is still employed in jewellery to signify any yellow stone. The true topaz was probably included by Pliny under the name _chrysolithus_. [Illustration: FIG. 72.—Topaz Crystal.]