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74
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PRECIOUS STONES.
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given off
to the extent of 20 to 24 per cent. With moderate heating no loss of
weight, but merely change of colour, occurs, the bulk of the stone
remaining unaltered, and consequently its specific gravity suffering no
increase or diminution. The sherry-coloured, the brown, and the other
tinted topazes, which are susceptible of being " pinked " by heat,
exhibit a very curious phenomenon during the operation. When a suitable
stone is packed in magnesia or other inert material, and heated in a
crucible, the specimen, if removed before it is cold and laid
upon a white surface, shows scarcely any trace of colour ; but after a
little time, when the stone has acquired the temperature of the air,
the desired pink hue makes its appearance. If the temperature reached
has not been sufficiently high, a salmon tint, or a hue like that of a
drop of blood mingled with much water, is obtained instead of a
rose-petal pink. What the cause of these changes of colour is remains
doubtful : it may be a change in the molecular or physical condition
of some minute trace of a coloured constituent in the topaz, or it may
be an actual chemical change. Anyhow the colour of topaz is a very
unstable one, for light, or at least the solar rays, soon exerts a
bleaching effect on many pale-coloured specimens ; so that the fine
suite of wine-hued Russian crystals, collected by Colonel de
Kokscharow, and now in the British (Natural History) Museum, is kept
shrouded from the light of day.
Topaz
occurs in several Scotch and Irish, and in some English, localities—St.
Michael's Mount in Cornwall, may be named amongst the latter. Villa
Rica, Minas Novas in Minas Geraes in Brazil, Flinders Island, and many
places in the United States, as well as several Siberian localities,
furnish splendid specimens of colourless and coloured topaz. The white
topazes from Minders Island are less brilliant than those from Brazil.
Good topazes come from Pegu and Ceylon, and they have been found in
Australasia. A magnificent deep blue topaz was found in Ceylon in 1899
: when cut, it weighed no less than 355 carats. The topazios of the
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