Encyclopædia Britannica, Ninth Edition/Topaz
TOPAZ. It appears that the stone described by ancient writers under the name of τοπάζιος, in allusion to its occurrence on the island of Topazion in the Red Sea, was the mineral which we now know as the chrysolite or Peridote (q.v.). The topaz of modern mineralogists was unknown to the ancients. Topaz occurs either crystallized, in association with granitic rocks, or in the form of rolled pebbles in the beds of streams. The crystals are orthorhombic prisms, with a perfect cleavage parallel to the base, or transverse to the long axis of the prism. This cleavage is recognized by the lapidary as the "grain" of the stone. It is notable that crystals of topaz are commonly hemimorphic; in other words, the prisms are terminated by dissimilar faces. This hemimorphism is associated with the property of pyroelectricity Mineralogy, (vol. xvi. p. 376). The chemical composition of the topaz has given rise to much discussion, but the mineral is now generally regarded as a silicate of aluminium associated with the fluorides of aluminium and silicon. When strongly heated it suffers considerable loss of weight. Brewster, examining the topaz microscopically, detected numerous fluid cavities, whence he concluded that it had been formed in the wet way. Two of the fluids obtained from these cavities have received the names of brewsterlinite and cryptolinite. Some of the finest topazes are almost colourless, and may be occasionally mistaken for diamonds. The topaz, however, is inferior in hardness; it lacks "fire"; and it becomes electric when heated—a property not possessed by the diamond. Colourless topazes are known to French jewellers as gouttes d'eau, and in Brazil as pingas d'agoa—names which refer to the limpidity of the stone—while in England they pass in trade under the curious name of minas novas. The beauty of the stone is best developed when in the form of a brilliant. The topaz is cut on a leaden wheel by means of emery, and is polished with tripoli. Coloured topazes are usually either yellow or blue. The pleochroism of the stone is very marked: thus the colour of the sherry-yellow crystals from Brazil is resolved by the dichroiscope into brownish yellow and rose-pink. The colour is unstable, the yellow topaz especially being liable to suffer bleaching by exposure to sunlight. Hence the fine series of crystals of Siberian topaz from the Kochscharow collection, now in the British Museum, is carefully protected from light by means of opaque pasteboard caps. In 1750 a Parisian jeweller named Dumelle discovered that the yellow topaz of Brazil, when exposed to a moderate heat, assumed a rose-pink colour. It is generally believed that all the pink topaz occurring in jewellery owes its tint to artificial treatment. Formerly it was the practice to heat the stone in a sand-bath, but the change of colour is now generally effected by wrapping the stone in German tinder, which is then ignited. This "burnt topaz" is sometimes known to jewellers as "Brazilian ruby." In like manner the blue topaz occasionally passes under the name of "Brazilian sapphire," and the pale green as "aquamarine"—a name which is strictly applicable only to the sea-green beryl. The largest known cut topaz is a fine brilliant, weighing 368 carats, and termed the "Maxwell Stuart topaz."
The topaz is occasionally found in Britain, but usually in small crystals unfit for jewellery. It occurs in granite at St Michael's Mount in Cornwall, in Lundy Island, and in Arran, but the finest British specimens are obtained from the Highlands of Scotland. Ben-a-bourd, one of the Cairngorm group, yields good blue crystals. Topaz occurs in colourless and blue crystals in the granite of the Mourne Mountains in Ireland; and microscopic crystals are not uncommon in certain other granites. The famous topaz-rock of the Schneckenstein, in Saxony, yields pale yellow crystals of great beauty as mineralogical specimens, but not suited for cutting. The yellow Saxon topaz does not seem to change colour on exposure to heat. Some of the finest topaz comes from near Odon Tchelon, in Siberia; while the well-known deep-yellow crystals of Brazil occur near Villa Rica (Ouro Preto). Fine topaz, pale blue and colourless, is found, as rolled crystals, in Tasmania and on Flinders Island in Bass's Strait. It also occurs in the tin-drifts of New South Wales; and beautifully-formed limpid crystals, of small size, accompany stream-tin at Durango, in Mexico. Fine topaz fit for jewellery has recently been worked at the Platte Mountain, near Pike's Peak, Colorado. One stone, weighing 125 carats, has been described as being "as fine a gem as America has produced of any kind" (Kunz, 1885). Topaz also occurs in cavities in rhyolite at Nathrop and Chalk Mountain, Colorado, and in trachyte near Sevier Lake, Utah. It is likewise found in Arizona, in New Mexico, and at Stoneham, Maine.
Oriental topaz is the name sometimes given to yellow corundum, a mineral which is readily distinguished from ordinary topaz by its superior hardness and density. The yellow and smoky varieties of quartz, or cairngorm, are often known in trade as Scotch topaz, but these are inferior to true topaz in hardness and in density. The chief differences between the three may be thus expressed:—
|Scotch Topaz.||True Topaz.||Oriental Topaz.|