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. 



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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 gravity of quartz (2'65). The specific gravity of certain specimens of emerald from 

 Muzo has been found to be 2-67, while for Siberian aquamarine the values 2-68 to 2'75 are 

 given. Beryl will thus always float in methylene iodide, and when pushed beneath the sui-face 

 quickly rises again ; in liquid No. 4 with the specific gravity of quartz it slowly sinks. 

 This character affords a ready means by which it may be distinguished from certain stones 

 of similar appearance. 



With the exception of hydrofluoric acid, beryl is unattacked by acids. When a 

 fragment is heated before the blowpipe it becomes white and cloudy, and fuses, only with 

 difficulty, at the edges to a white, blebby glass. 



Different beryls differ greatly in appearance, especially in respect to colour and 

 transparency, some being cloudy and opaque, others beautifully clear and transparent ; all 

 possible gradations between these two extremes are known. The opaque variety, known as 

 common beryl, usually occurs as crystals in coarse-grained granite. Such crystals have been 

 known to measure 6 feet in length, and to weigh 2^ tons ; they are useless, however, for 

 gems, since, besides being opaque, they are usually of an unpleasing yellowish- or greenish- 



