Gems - Topaz
( Originally Published 1919 )
TOPAZ is the most popular yellow stone in jewellery, and often forms the principal stone in brooches or pendants, especially in old-fashioned articles. It is a general idea that all yellow stones are topazes, and all topazes are yellow ; but neither statement is correct. A very large number of yellow stones that masquerade as topaz are really the yellow quartz known as citrine. The latter is, indeed, almost universally called by jewellers topaz, the qualification 'Brazilian' being used by them to distinguish the true topaz. Many species besides those mentioned yield yellow stones. Thus corundum includes the beautiful 'oriental topaz' or yellow sapphire, and yellow tourmalines are occasionally met with ; the yellow chrysoberyl always has a greenish tinge. Topaz is generally brilliant-cut in front and step-cut at the back, and the table facet is sometimes rounded, but the colourless stones are often cut as small brilliants ; it takes an excellent and dazzling polish.
Topaz is a silicate of aluminium corresponding to the formula [Al(F,0H)ISiO4, 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, 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 sun-light, 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 , 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 con-temporaries 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, the term is still employed in jewellery to signify any yellow stone. The true topaz was probably included by Pliny under the name chrysolithus.
The symmetry is orthorhombic, and the crystals are prismatic in shape and terminated by numerous inclined faces, and usually by a large face perpendicular to the prism edge (Fig. 72). Topaz cleaves with great readiness at right angles to the prism edge ; owing to its facile cleavage, flaws are easily started, and caution must be exercised not to damage a stone by knocking it against hard and unyielding sub-stances. The dichroism of a yellow topaz is always perceptible, one of the twin colours being distinctly more reddish than the other, and the phenomenon is very marked in the case of stones the colour of which has been artificially altered to pink. The values of the least and the greatest of the principal indices of refraction vary from 1.615 to 1.629, and from V625 to 1.637, respectively, the double refraction being about 0.010 in amount, and positive in sign. The high values correspond to the altered stones. The specific gravity, the mean value of which is 3.55 with a variation of 0.05 on either side, is higher than would be expected from the refractivity. A cleavage flake exhibits in convergent polarized light a wide-angled biaxial picture, the `eyes' lying outside the field of view. The relation of the principal optical directions and the directions of single re-fraction to the crystal are shown in Fig. 27. The hardness is 8 on Mohs's scale, and in this character it is surpassed only by chrysoberyl, corundum, and diamond. Topaz is pyro-electric, in which respect tourmaline alone exceeds it, and it may be strongly electrified by friction.
Although the range of refraction overlaps that of tourmaline, there is no risk of confusion, because the latter has nearly thrice the amount of double refraction (cf. p. 29). Apart from the difference in refraction, a yellow topaz ought never to be confused with a yellow quartz, because the former sinks, and the latter floats in methylene iodide. The same test distinguishes topaz from beryl, and, indeed, from tourmaline also.
Judged by the criterion of price, topaz is not in the first rank of precious stones. Stones of good colour and free from flaws are now, however, scarce. Pale stones are worth very little, possibly less than 4S. a carat, but the price rapidly advances with increase in colour, reaching 20S. for yellow, 80s. for pink and blue stones. Since topazes are pro-curable in all sizes customary in jewellery, the rates vary but slightly, if at all, with the size.
Topaz occurs principally in pegmatite dykes and in cavities in granite, and is interesting to petrologists as a conspicuous instance of the result of the action of hot acid vapours upon rocks rich in aluminium silicates. Magnificent crystals have come from the extensive mining district which stretches along the eastern flank of the Ural Mountains, and from the important mining region surrounding Nertschinsk, in the Government of Transbaikal, Siberia. Fine green and blue stones have been found at Alabashka, near Ekaterinburg, in the Government of Perm, and at Miask in the Ilmen Mountains, in the Government of Orenburg. Topazes of the rare reddish hue have been picked out from the gold washings of the Sanarka River, Troisk, also in the Government of Orenburg. Splendid pale-brown stones have issued from the Urulga River, near Nertschinsk, and good crystals have come from the Adun-Tschilon Mountains. Kamchatka has produced yellow, blue, and green stones. In the British Isles, beautiful sky-blue, waterworn crystals have been found at Cairngorm, Banffshire, in Scotland, and colourless stones in the Mourne Mountains, Ireland, and at St. Michael's Mount, Cornwall. Most of the topazes used in jewellery of the present day come from either Brazil or Ceylon. Ouro Preto, Villa Rica, and Minas Novas, in the State of Minas Geraes, are the principal localities in Brazil. Numerous stones, often waterworn, brilliant and colourless or tinted lovely shades of blue and wine-yellow, occur there; reddish stones also have been found at Ouro Preto. Ceylon furnishes a profusion of yellow, light-green, and colourless, waterworn pebbles. The colourless stones found there are incorrectly termed by the natives water-sapphire,' and the light-green stones are sold with beryl as aquamarines ; the stones locally known as ` king topaz' are really yellow corundum (cf. p. 18 1). Colourless crystals, some-times with a faint tinge of colour, have been discovered in many parts of the world, such as Ramona, San Diego County, California, and Pike's Peak, Colorado, in the United States, San Luis Potosi in Mexico, and Omi and Otami-yama in Japan.