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TOURMALINE.
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ancients
was our chrysolite and peridot, not the stone now called topaz, which
was not known as a distinct stone until comparatively modern timei.
The topaz (pitdâh) of Aaron's breastplate was probably a peridot.
The
commercial value of the topaz is small and variable. Very
richly-coloured specimen stones, suitable for pendants may be bought
for a pound or a few pounds ; they are often sold by the ounce, not by
the carat.
Tourmaline.
The
tourmaline is marked out from all other precious stones by a very
complex chemical constitution, and by a very interesting optical
structure. Its hardness, 7.3 to 7.5, suffices to protect it from wear,
while the range and quality of the colours which it exhibits commend it
to those persons who appreciate the artistic value of jewellery in
which other stones besides those which are well known and popular form
dominant elements.
All
the minerals called by the names " indicolite " (blue), " rubellite "
(red), " schorl " (black), and " achroite " (colourless) form but one
species—tourmaline. These differences of colour are accompanied by
differences of composition, so that we have a series of varieties of
tourmaline, in which, while the proportion of silica is fairly
constant, the bases consist of the oxides of iron, magnesium, sodium,
manganese, and aluminium in differing proportions. Water is also
present, and sometimes lithia and potash. To give some notion of the
chemical complexity of the tourmaline we may cite an analysis by
Rammelsberg of a green Brazilian stone of specific gravity 3.107 :
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