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INTRODUCTION.
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represents the specific gravity of the gem which has been put to the proof.
It
is often very useful to know the specific gravity of gems, as a means
of determining their quality, and to prevent the danger of their being
changed for others, similar in appearance but very different in value ;
as, for instance, the diamond and the jargoon.
In
the case of cut and polished gems it is therefore of great use in
ascertaining the class to which they belong, without the possibility of
injuring them by the otherwise necessary operation of filing.
The
comparison of the specific weight of gems was, according to Emmanuel,
known to, and practised by, the ancients, and certainly in the Indies,
for many centuries, in order to ascertain the quality of a precious
stone.
BRIGHTNESS, COLOUR, AND TRANSPARENCY.
Gems
acquire more or less brightness, lively, varied colouring, and
transparency, in proportion to their single or double refraction, and
to the polarization of light.
The brightness of gems may be distinguished as follows :
I. Adamantine brightness; that which gives out the splendour peculiar to the diamond.
II. Vitreous brightness; that which resembles glass.
III. Resinous brightness; that which shines as if the surface had been rubbed with oil.
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