TOPAZ, a mineral of some importance as a gemstone, but apparently h'aving no practical application. It usually occurs in association with cassiterite in connection with granitic rocks, and it is thus often a valuable indicator of the presence of tin-ore. It is usually found as bright, well-developed crystals, or as clear water-worn pebbles, though gianular masses of topaz-rock are also known. The crystals belong to the orthorhombic system and are usually prismatic in habit, with several dome and pyramid faces (figs. I and 2). The prism faces (M and 1) are striated vertically, in contradistinction to quartz in which the prism faces are striated horizontally. Doubly terminated crystals (fig. I) are extremely rare ; usually the crystals are attached at one end in the rock cavities in which they grew, and when detached from the matrix they often break along a flat surface parallel to the basal plane (c in fig. 2). This perfect cleavage is a very important character, and enables topaz to be distinguished at sight from other minerals of similar appearance. The cleavage flakes when examined in convergent polarized light show a good biaxial inter ference-figure. Chemically, topaz is a fluo-silicate of aluminium in which the fluorine is in part replaced isomorphously by hydroxyl, the formula then being (Al[F,011])2SiO4. With this range in chemical composition there is a slight variation in the specific gravity (3.574 to 3.523) and in the optical constants (refractive indices and optic axial angle), but the difference is of little practical importance.
The crystals are often perfectly colourless and water-clear, but owing to the presence of traces of various colouring matters they may show a wide range of colours—red, yellow, brown, green, blue. Further, it is an interesting fact that some of these colours
are by no means stable. The fine brown crystals from pegmatite veins in Transbaikalia, Siberia, and the smaller wine-yellow crystals occurring in rhyolite in Colorado and Utah, fade on ex posure to light ; and the sherry-yellow crystals from Brazil assume a fine pink colour when they are heated. The deceptive trade names—"Scotch topaz," "Spanish topaz," or "occidental topaz" for yellow quartz (citrine), and "oriental topaz" for yellow corun dum ("yellow sapphire") represent three distinct minerals whose characters and differences are set out in the following table: The sherry-yellow crystals of topaz, which in the past have been cut and extensively used in jewellery, are all from the neigh bourhood of Ouro Preto, in Brazil; and it is this material that has supplied the pink ("burnt") topaz. Brazil also supplies colourless and pale-blue topaz. Good crystals of pale blue and green colours have come from the Ural Mountains and from Nerchinsk in Siberia. Colourless water-worn crystals and clear pebbles resembling rock-crystal are abundant in the alluvial de posits of tin-ore in Northern Nigeria, and small colourless crystals are well known from the Cornish tin mines. A large rough crystal weighing 137 lb. of opaque topaz, from a felspar quarry in Nor way, is shown in the mineral collection of the British Museum (Natural History), with many other interesting crystals from various localities. (L. J. S.)