. Precious stones, a popular account of their characters, occurrence and applications, with an introduction to their determination, for mineralogists, lapidaries, jewellers, etc. With an appendix on pearls and coral. Precious stones; Pearls; Corals. 60 GENERAL CHARACTERS OF PRECIOUS STONES colour in different directions ; these precious stones behave in this respect like their glass imitations. Taking refraction as the basis of classification, we have seen that all minerals can be divided into two groups, namely, those which possess single refraction and those which possess double refraction.
. Precious stones, a popular account of their characters, occurrence and applications, with an introduction to their determination, for mineralogists, lapidaries, jewellers, etc. With an appendix on pearls and coral. Precious stones; Pearls; Corals. 60 GENERAL CHARACTERS OF PRECIOUS STONES colour in different directions ; these precious stones behave in this respect like their glass imitations. Taking refraction as the basis of classification, we have seen that all minerals can be divided into two groups, namely, those which possess single refraction and those which possess double refraction. The former group will include those minerals which are not possessed of dichroism, while all dichroic minerals fall into the latter. Thus amorphous substances and those which crystallise in the cubic system are characterised by single refrac- tion and absence of dichroism, while all coloured minerals included in the remaining five crystal systems are dichroic, and all without exception are doubly refracting. The phenomenon of dichroism then, furnishes us with additional aid in distinguishing singly from doubly refracting stones. A body showing this character to even the feeblest degree cannot be amorphous nor can it be a cubic mineral. The apparent absence of dichroism however must be considered only as negative evidence in favour of single refrac- tion, since dichroism may be present but so feeble as to be detected only with difficulty. It has been shown above that the phenomena of double and of single refraction enables us to distinguish a ruby from a red spinel, and this is made still more easy from the fact that the hexagonal ruby is distinctly dichroic, while the cubic spinel does not possess this property. In the same way an imitation ruby of red glass could not be confused with the genuine stone, since the former, being amorphous, is not dichroic and shows the same colour in all directions. The detection of dichroism usually requires the use of a special instrument. The
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