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Appendix

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298 Specific Gravity of Gem Minerals
REFRACTION
The refractive indices of the more important pre­cious and semi-precious stones are given in the following table, the values for singly refracting stones being indicated by n, and the greatest and least values for doubly refracting stones by n y and n z, respectively. In both cases the values apply to the middle rays of the spectrum. The strength of the double refraction of each stone is indicated by d-n y—n z, that is, by the difference between the greatest and the least refractive indices of the stone.
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