. 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. QUARTZ (AMETHYST) 483. Fig. 86. Sceptre-quartz. quartz then, there is a long, slender, usually colourless, transparent, or cloudy prism of quartz, attached to the end of which in parallel position is a thicker quartz crystal, which is usually transpai-ent and of a violet colour. Crystals of amethyst sometimes attain a considerable size, the largest
. 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. QUARTZ (AMETHYST) 483. Fig. 86. Sceptre-quartz. quartz then, there is a long, slender, usually colourless, transparent, or cloudy prism of quartz, attached to the end of which in parallel position is a thicker quartz crystal, which is usually transpai-ent and of a violet colour. Crystals of amethyst sometimes attain a considerable size, the largest known being over a foot in length. These very large crystals are rarely quite transparent or uniformly coloured, and are therefore unsuitable for gems; but there is an abundance of crystals which fulfil these conditions, and which are large enough to supply the market with any amount of rough material. With respect to the mode of occurrence of amethyst, we have already seen that crystals of elongated prismatic habit usually occur, like rock- crystal, on the walls of crevices and joints in granite, gneiss, and other rocks. Crystals in which only the hexagonal pyramid or three-faced cube-like termination is developed, have a different mode of occurrence. These usually line amygdaloidal, that is to say, almond-shaped, cavities in a black igneous rock, to which the name melaphyre is applied. The cavities were formed, in their efforts to escape, by the steam and gases imprisoned in the molten igneous rock, and have remained after the solidification of the rock. As long as the rock preserved its fresh, unaltered condition, these cavities remained empty, but with the weathering and alteration of the rock the cavities become wholly or partially filled with secondary minerals. The alteration process is set up by the percolation through the rock of water, which, becoming charged with carbon dioxide and other acids, is enabled to dissolve out many of the constituents of the rock, which a
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