. Pietre preziose, un popolare conto dei loro caratteri, il verificarsi e applicazioni, con un'introduzione alla loro determinazione, per mineralogists, lapidaries, gioiellerie, ecc. Con una appendice su perle e corallo. Pietre preziose, perle, coralli. BERYL 307 piramide del primo ordine, mentre in Fig. e ci sono due piramidi esagonale del primo oi-der, uno del secondo ordine e di una piramide dihexagonal in combinazione con un prisma esagonale e il piano basale. Questi più complicati i cristalli sono più caratteristica di acquamarina. I cristalli sono sia attaccato alla matrice da una estremità, in

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. 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. BERYL 307 pyramid of the first order, while in Fig. e there are two hexagonal pyramids of the first oi-der, one of the second order, and a dihexagonal pyramid in combination with a hexagonal prism and the basal plane. These more complicated crystals are more characteristic of aquamarine. The crystals are either attached to the matrix by one end, in which case they often form beautiful druses, or they are embedded in it, and are then developed regularly in all dii-ections. In the former case, the free end alone bears regular crystal-faces, while in the latter both ends are developed; these terminal faces are, however, sometimes small and in-egular. The cleavage of beryl is not an important character, crystals of this mineral cleaving only indistinctly in certain directions. The mineral is brittle, and its fracture conchoidal. {"^-^^ (^^. Fig. 62. Crystalline forms of beryl (emerald and aquamarine). With respect to its hardness it stands a little above quartz, but below topaz, the degree of its hardness being represented on the scale by H = 7|^; this is rather low for a precious stone, and beryl is, in fact, one of the softer of the more valuable gems. The different varieties of the mineral show small differences in hardness among themselves, the Colombian emerald being, for example, a little softer than the Siberian aquamarine. Although the hardness of this stone is not great, it is sufficient to render it susceptible of a fine polish, which, however, is not retained for as long as is the case with harder stones. The specific gravity of beryl, like its hardness, is rather low, its mean value being 2*7. That of precious beryl varies between 2"67 and 2"75, being always slightly higher than the specific gravit

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• bookcentury1900
• bookdecade1900
• booksubjectpreciousstones
• booky