. 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. THE GARNET GROUP 347 Fig. 29i, are usually delicately striated in the direction of their length. Sometimes the faces of the rhombic dodecahedron are more largely developed than are the truncating faces (Fig. 296); at other times the i-everse is the case, and the rhomb-shaped faces are small. These truncating faces belong to the icositetrahedron (Fi
. 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. THE GARNET GROUP 347 Fig. 29i, are usually delicately striated in the direction of their length. Sometimes the faces of the rhombic dodecahedron are more largely developed than are the truncating faces (Fig. 296); at other times the i-everse is the case, and the rhomb-shaped faces are small. These truncating faces belong to the icositetrahedron (Fig. 69c), a simple form, uncombined with others, frequently taken by garnet. The edges of the rhombic dodecahedron are in many cases not- only truncated by the icositetrahedron, as in Fig. 69b, but the edges of. i. c. Fig. 69. Crystalline forma of giu-net. intersection of these two forms are further truncated by delicately stiiated faces, the result being a form like Fig. Gfdd. This second series of faces are those of a hexakis-octahedron, a solid bounded by forty-eight faces, which is the greatest number possible on any single uncombined form. The simple hexakis-octahedron uncombined with other forms has not been observed in garnet. The mineral rarely takes a form other than those mentioned; those forms having a lesser number of faces, such as the octahedron and the cube, which, as a rule, are commonest in other minerals, are rarely seen in garnet. The cleavage of garnet is more imperfect than in most other minerals ; the fracture is sub-conchoidal to uneven. It is fairly hard, but this character varies in different varieties. All red garnets, which are the varieties chiefly used as gems, are harder than quartz but less hard than topaz, that is to say, for red garnets H = 7 — 8. The hardness of some green garnet is rather less ; the demantoid, for example, which is sometimes used as a gem, has a hardness of Q^ only, and is scratched by quartz ; it is sufficiently hard, howeve
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