. Pierres précieuses, un compte rendu populaire de leurs personnages, l'occurrence et d'applications, avec une introduction à leur détermination, pour les minéralogistes, lapidaires, bijoutiers, etc. avec un appendice sur les perles et le corail. Pierres précieuses ; Perles ; les coraux. Fig. 61. Formes cristallines de chrysobéryl. clair alors que d'autres troubles et opaques. La transparence, même des meilleurs spécimens, est, cependant, généralement qu'apparente lorsque les histoires ont été taillées et polies, depuis le minéral est plus fréquente dans la nature que l'eau usée et apparemment cailloux. opaque Il est facile à l'aide de l'polariscop

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. 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. Fig. 61. Crystalline forms of chrysoberyl. clear while others are cloudy and opaque. The transparency, even of the clearest specimens, is, however, usually only apparent when the stories have been cut and polished, since the mineral is found most frequently in nature as water-worn and apparently opaque pebbles. It is easy by means of the polariscope to demonstrate the doubly refracting character of transparent fragments of chrysoberyl. The double refraction of this mineral is not, however, very strong, the two indices of refraction having been determined as 1'756 and l"74i7 respectively. The strength of refraction possessed by chrysoberyl is thus also rather small, and about the same as that of corundum. The refractive indices of chrysoberyl for the various colours of the spectrum differ from each other but little, so that the dispersion produced is slight and no prominent play of prismatic colours is seen. Chrysoberyl shows only a limited range of colour; in Brazil, which is the most important locality of the mineral, it varies from pale yellowish-green to golden yellow and brownish-yellow. The Uralian crystals are of an intense green colour, varying in shade from grass-green to emerald-green. There are thus two varieties of chrysoberyl to be recognised, the one of a pale yellowish-green colour, considered as chrysoberyl proper, and the darker emerald-green variety, which is distinguished by the name of alexandrite. The first named is the more abundant and typical variety, the second being comparatively rare and of less importance. CHRYSOBERYL PROPER AND CYMOPHANE. The common variety of chrysoberyl is typically pale in colour. Green strongly tinged with yellow (Plate XII., Figs. 10 and 11), more or less bright oliv

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