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22 PRECIOUS STONES.
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very
various kinds have unfortunately become known by the same name when
they are of one colour. For example, a red Corundum is Ruby, while a
blue crystal of the same mineral is a Sapphire, but a red Spinel may
also be sold as a Ruby ; an expert eye can usually distinguish a
difference in such a case, but it is not absolutely reliable.
b. Lustre is of several different kinds, usually described as—
1. Metallic.
2. Adamantine.
3. Vitreous.
4. Greasy.
5. Resinous.
6. Silky.
7. Pearly.
The lustre may be of the varying degrees of splendent, shining, glistening or glimmering.
By
far the most important in the crystallised gem stones is the adamantine
lustre, and that we wish to find in the splendent degree—in other
words, the ideal is to have the stones of as great a brilliance of
lustre as possible, and hence stones which do not reach this standard
are thought less of. A high lustre is almost an essential of a true gem
stone at present, though there are signs of a coming appreciation of
the more modest appearance of some of the less splendent minerals.
B. When the light is transmitted there are three conditions to be observed :—
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a. Diaphaneity. b. Refraction. c. Polarisation.
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