Page:Handbook of Precious Stones.djvu/89

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TOPAZ.
73

Brazil showed two images, one rose pink and the other brownish yellow, in the dichroiscope; after heating, the same crystal gave a stronger pink colour and a dull white. The colours of topaz are many and beautiful; the rose pink (often called burnt topaz) is commonly obtained by heating the richly coloured wine-yellow or amber-yellow crystals, but occasionally occurs in natural specimens. Blue and pale-green topazes are sometimes found of large size, and are more brilliant than similarly tinted beryls. Colourless topazes vary a good deal in purity of hue and fire; those from Brazil are often of remarkable whiteness, and show dazzling reflections of pure white light when properly cut. The polish which the topaz takes is very high, and the surface of cut specimens is exceedingly smooth and slippery to the touch.

The specific gravity of the topaz, even in perfectly flawless and transparent specimens, ranges between rather wide limits, so far as the colourless specimens from different localities are concerned. The coloured specimens show a much smaller variation.

Topaz, white . . . 3 -597

. . . 3-595

. . . 3-585

. 3-572

Topaz, rose pink . . 3 '534

. 3-533

,, sherry yellow . 3 '539

blue 3-541

The topaz is one of the few precious stones containing the element fluorine; it may be regarded as a silicate of alumina, in which part of the oxygen of the silica is replaced by fluorine. The analysis of topazes from different localities points to a composition which may be represented in 100 parts by these figures:

Silicon . . . . 15-5 I Oxygen .... 36 '8 Aluminium . . . 30'2 I Fluorine . . . . 17 '5

A little water is always present in topaz; it is probable that the hydroxyl of this replaces a part of the fluorine. Such replacement helps to account for slight differences in the physical characters of topaz from different localities. When strongly heated, topaz not only changes in colour, but loses considerably in weight ; hydrofluoric acid, fluoride of silicon, and fluoride of aluminium being

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