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SYSTEMATIC DESCRIPTION OF PRECIOUS STONES 



BERYL. 



The mineral species beryl includes, besides the emerald and the aquamarine, obher 

 precious stones of less importance, which are referred to generally by jewellers as beryl. 

 The different varieties of beryl differ only in colour ; their other characters are identical, 

 just as ruby and sapphire are mere colour- varieties of the mineral species corundum. It 

 will be convenient to consider the specific characters before passing to a more detailed 

 description of each colour- variety. 



The oxide alumina, which enters so largely into the composition of corundum, spinel, 

 and chrysoberyl, is also present in beryl, but in smaller amount and in combination with 

 silica and beryllia. The oxide beryliia, so called on account of its presence in "beryl, is also, 

 as we have seen, a constituent of chrysoberyl. Beryl is thus a silicate of the metals 

 aluminium and beryllium, the chemical composition of which is expressed by the formula 

 SBeO.AIgOg.GSiOg, and the percentage composition by, silica (SiOg) = 66'84, alumina 

 (AI2O3) = 19-05, beryllia (BeO) = 14-11. 



In several analyses of this mineral the presence, in small amounts, of water, iron, 

 alkalies, chromic oxide, and other substances have been determined, while in some beryls, as, 

 for example, the beautiful emerald from Muzo in Colombia, South America, traces of 

 organic matter have been found. The result of the analysis of this stone made by Lewy, 

 together with an analysis by Penfield of aquamarine from Adun-Chalon, iii Siberia, is given 

 below. Chromic oxide, which is absent from this specimen of aquamarine and exists as a 

 trace in the emerald, is sometimes present to the extent of 3 per cent. 



Emerald Aquamarine 



(Colombia). (Siberia). 



Silica (SiOj) 67-85 6(5-17 per cent. 



Alumina (AlgOg) 17-95 20-39 



Beryllia (BeO) 12 4 11-50 



Chromic oxide (Ct^O^) . . . trace — 



Ferrous oxide (FeO) . . — 0-69 



Magnesia (MgO) .... 0-9 — 



Soda CNa.fi) 0-7 0-24 



Lithia (LijO) — trace 



Water (Rfi) 1-66 1-14 



Organic matter 0-12 — 



Beryl crystallises in the hexagonal system. The crystals (Fig. 62, a to e) are usually 

 rather long, six-sided prisms with smooth faces, terminated in many cases, and nearly always 

 in emerald, by a single plane at right angles to the faces of the prism (Fig. 62 a), this being 

 known as the basal plane. Not infrequently the edges of the hexagonal prism are 

 truncated by the faces of a second hexagonal prism, and these again by a twelve-sided 

 prism ; the resulting form, though in reality a prism, bounded by many small faces 

 (P'ig. 62 d), has the appearance of a longitudinally-striated cylinder. For this reason the 

 prism faces of beryl are usually striated in the direction of their length, that is, parallel to 

 their mutual intersections. Moreover, in many cases, the crystals are terminated not only 

 by the basal plane but also by six-sided and twelve-sided pyramids in combination with the 

 prism. Fig. b shows a hexagonal pyramid of the second order, and Fig. c a hexagonal 



