. 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. 262 SYSTEMATIC DESCRIPTION OF PRECIOUS STONES is to the end of the crystal; in combination with these three pyramids are the basal planes and a nan'ow hexagonal prism. The frequent repetition or oscillation between pyramids of this kind gives rise to horizontal striations on the faces, such as is shown inj^ In Fig. 53, i, a combination of two hexag
. 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. 262 SYSTEMATIC DESCRIPTION OF PRECIOUS STONES is to the end of the crystal; in combination with these three pyramids are the basal planes and a nan'ow hexagonal prism. The frequent repetition or oscillation between pyramids of this kind gives rise to horizontal striations on the faces, such as is shown inj^ In Fig. 53, i, a combination of two hexagonal bipyramids with the hexagonal prism and an acute rhombohedron is shown. As mentioned above, crystals of corundum occur in one of two habits, that is with either the prism or the hexagonal pyramids predominating. The former is the more characteristic for red corundum or ruby, and the latter for blue corundum or sapphire. A crystal of ruby is shown in Plate I., Fig. 5, and a sapphire crystal in Fig. 7 of the same plate, both stones being represented in their natural J. h. Fig. 53. Crystalline forms of corundum {a—d, ruby ; c—i, sapphire). Twin intergrowths are often met with in cloudy corundum, but less frequently in transparent material suitable for use as gems. These are of two kinds : in the one, large numbers of thin plane lamellae, parallel to a face of the primitive I'hombohedron, occupy twin positions in the crystal, as indicated in Fig. 53, a; in the other, illustrated by h of the same figure, the lamellae are parallel to the basal planes. Owing to this cause the faces of crystals often bear fine striations, which differ in direction in the two kinds of twin intergrowth. In the first, the striations are present on the basal plane in three sets of parallel lines intersecting at 120° or 60°, and on the prism and rhombohedron faces in two sets intersecting at an oblique angle. In the other kind of twin intergrowth, the basal planes are smooth, while the
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