. 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. 73. Crystalline forms of idocrase. pi'ism. The prism-taces are sometimes very numerous, and when this is the case the faces are narrow and the crystal almost cylindrical. The basal planes perpendicular to the prism-faces are usually moderately large ; between the basal plane and the prism-faces there are faces of a square pyramid (Fig. 73a), o
. 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. 73. Crystalline forms of idocrase. pi'ism. The prism-taces are sometimes very numerous, and when this is the case the faces are narrow and the crystal almost cylindrical. The basal planes perpendicular to the prism-faces are usually moderately large ; between the basal plane and the prism-faces there are faces of a square pyramid (Fig. 73a), or of a square pyramid in combination with an eight-sided pyramid (Fig. 736). These faces are usually very small in size, but sometimes extremely numerous ; indeed, crystals of idocrase have been met with which are richer in faces than crystals of any other mineral. Idocrase shows only a suggestion of cleavage; the fracture is imperfectly conchoidal to uneven. The mineral is brittle and the hardness (H = 6J) is rather less than that of quartz. It can be fused before the blowpipe with moderate ease,. and, after being fused, is decomposed by acids; before fusion it is unattacked by acids. The specific gravity varies between 3'3 and 3'5 according to the cheniical composition. The brown crystals are rather heavier than the green ; thus the specific gravity of brown crystals from Vesuvius has been determined to be 3'45, and that of green crystals from the Ala valley to vary between 3"39 and 3"43. The mineral, therefore, sinks in pure methylene iodide, but floats in the heaviest liquid (sp. gr. = 3-6). In the matter of transparency different specimens of idocrase may differ widely. The majority of crystals are only translucent to semi-transparent, the free end of an attached crystal being frequently more transparent than the other. Only transparent or very translucent crystals are cut as gems : by polishing they acquire a very good lustre, which is- vitreous in character, th
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