. 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. 8i5. Crystalline forms of quartz. More frequently than not it contains impurities of various kinds, a fact to which the variety in colour of the mineral is due. Quartz differs from opal, another mineral which consists lai'gely of silica, in that it is completely free from water. Quartz differs also from opal in another impoi'tant respect, name
. 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. 8i5. Crystalline forms of quartz. More frequently than not it contains impurities of various kinds, a fact to which the variety in colour of the mineral is due. Quartz differs from opal, another mineral which consists lai'gely of silica, in that it is completely free from water. Quartz differs also from opal in another impoi'tant respect, namely, in that it is not amorphous but crystallised. Very finely developed crystals of quartz, usually with brilliant faces, are extremely common. They belong to the hexagonal system (Figs. 85a —J), and almost without exception take the form of regular six-sided prisms, the faces of which are characterised by the presence of very distinct striations perpendicular to the edges of the prism. The latter is terminated at one end (Figs. 85& - d), and in completely developed crystals (Fig. 85a) at both ends, by a six-sided pyramid, the faces of which intersect the prism-faces in horizontal edges. In addition to these common faces there are often others of small size (Figs. 536 - d), the arrangement of which indicates that the symmetry of quartz is that of the ti'apezohedral-tetartohedral division of the hexagonal system. The liabit of quartz crystals is rather variable. In many cases the prism-faces are elongated, as in the forms represented in the figures ; but sometimes they are short or even completely absent, and in the latter case the crystal becomes a double six-sided pyramid. The faces of this pyramid may be all of the same size, but more frequently large faces alternate with small ones, so that there are three large and three small faces (Figs. 85b —d). Owing to irregular. Please note that these images are extracted from scanned page images that may have been digitally e
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