. 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. sm SYSTEMATIC DESCRIPTION OF PRECIOUS STONES Topaz crystallises in the rhombic system; all crystals have certain features in common, but show differences in habit. A combination of two rhombic prisms forming elongated, eight-sided columns, often deeply striated parallel to their length, is almost invariably to be seen. The terminal faces differ acc
. 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. sm SYSTEMATIC DESCRIPTION OF PRECIOUS STONES Topaz crystallises in the rhombic system; all crystals have certain features in common, but show differences in habit. A combination of two rhombic prisms forming elongated, eight-sided columns, often deeply striated parallel to their length, is almost invariably to be seen. The terminal faces differ according to the locality from which the crystal comes. As a rule, they are developed regularly at one end only of the crystal, the other end having been attached to the matrix in the drusy cavity in which the crystal grew. A few forms taken by topaz crystals are shown in Pig. 66, a to i ^ 'til Jji I:ii5i!l 1 ; b. c. Fig. 66. Crystalline forms of topaz. d. Pig. 66 «, and Plate XIII., Figs. 2 and 4, show a simple form of crystal especially characteristic of topaz from Brazil and Asia Minor ; here the only terminal faces are those of a rhombic octahedron or pyramid. In Pig. 66 b, the faces of this rhombic octahedron are small, and two large dome faces, give a roof-like termination to the crystal; this habit is characteristic of topaz crystals from the Adun-Chalon Mountains, near Nerchinsk, in Siberia. Crystals from Mursinka, in the Urals (Fig. QQ c, and Plate XIII., Fig. 1), have, in addition to these faces, a largely developed basal plane at right angles to the prism faces. The crystal shown in Fig. QQ d, is a combination of two prisms, three rhombic octahedra, a dome, and the basal plane; this habit is characteristic of crystals from Schneckenstein in Saxony (Plate XIII., Fig. 3). More complicated crystals, with a much larger number of faces, are to be found at other localities ; the examples cited are sufficient, however, to give a general idea of the crystalline forms of to
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