TOPAZ
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,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.]
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
substances. 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 1·625 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 refraction 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 procurable 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, Troitsk, 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. 181).
Colourless crystals, sometimes 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.