The Topaz. 125
The
cleavage is basal and very perfect, having a highly polished surface on
the cleavage planes; the lustre is vitreous. This stone is found of
many colours : the fine pink so often observed in topazes in many
articles of jewellery is never natural, but is produced by exposing a
brownish-yellow topaz to a low red-heat in a sand-bath. The experiment
can also be tried by wrapping the stone up in German tinder, bound with
thin iron wire, and then setting fire to it, or with the blowpipe.
Those only of the peculiar brown colour described can acquire this pink
hue; the pale yellow stones become perfectly white under the treatment.
The colour thus acquired is permanent. The topaz becomes strongly
electric by heat, friction, and pressure, retaining and continuing in
that state for several hours. This characteristic is so marked as to
afford an easy method of ascertaining its identity; the application of
this test alone would at once prevent the stone being mistaken for a
chrysoberyl or a yellow sapphire. Before the blowpipe, it is infusible
on charcoal, but in a very strong heat blisters form on the surface,
which break as soon as they rise. It fuses with borax into a clear
glass, and becomes blue with cobalt solution.
If sulphuric acid be applied to this stone it yields hydrofluoric, but muriatic does not affect it.
The
topaz is found in almost every part of the world, generally in the
granite and gneiss rocks which contain fluor spar, but it varies in
colour and aspect in almost every district. Those from Villa Rica in
Brazil have a
