. 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. TOURMALINE 365 shown most conspicuously by the crystals lettered i, c, and d, there being many more faces present on the upper than on the lower ends. Further, the hemimorphic development is shown not only by the number and arrangement of the terminal faces, but also by that of the prism faces. For example, in Fig. 70a, the prism has only three fac
. 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. TOURMALINE 365 shown most conspicuously by the crystals lettered i, c, and d, there being many more faces present on the upper than on the lower ends. Further, the hemimorphic development is shown not only by the number and arrangement of the terminal faces, but also by that of the prism faces. For example, in Fig. 70a, the prism has only three faces instead of six, the number usually present on crystals belonging to the hexagonal system ; again, in Figs, h to e, instead of a prism of twelve faces there are only nine, this being a combination of a hexagonal with a trigonal prism. This feature is so characteristic of crystals of tour- maline that they may be recognised with certainty by it alone, even when no terminal faces are present. The faces of the prism are usually more or less deeply striated in the direction of their length ; that is to say, parallel to the principal trigonal axis of the crystal, as is represented in the figures. This striation is specially prominent when more than nine prism faces are. 6, o. d-. Fig. 70. Crystalline forms of tourmaline. developed, the crystals then having the appearance of cylinders, in which, however, a triangular arrangement can still be detected (Plate XV., Figs. 8 and 9). The terminal faces are usually smooth and not striated; some, however, are rough and dull. The form of crystals of tourmaline varies somewhat according to the locality at which they occur, but there is no essential difference between them. Of the diagrams of Fig. 70, a represents a brown crystal from Ceylon, h and c two green crystals from Brazil, d a red crystal from Shaitanka in the Urals, and e a rose-red crystal from the island of Elba. These are all forms taken by precious, transparent varieties of tour
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