CHRYSOBERYL 



301 



Chrysoberyl has no distinct cleavage but a conchoidal fracture. It is brittle and very 

 hard (H = 8|), and has a specific gravity of 3-68 to 3-78. It is unattacked by acids and 

 infusible before the blowpipe. When rubbed it becomes positively electrified and retains 

 its charge for several hours. 



Ihe lustre of chrysoberyl is vitreous or slightly inclined to greasy in chai-acter; the 

 mineral takes, and on account of its great hardness retains unaltered, a very brilliant polish. 

 With regard to transparency the mineral is very variable, some specimens being beautifully 



Fig. 61. Crystalline forms of chrysoberyl. 



clear while others are cloudy and opaque. The transparency, even of the clearest 

 specimens, is, however, usually only apparent when the stories have been cut and polished, 

 since the mineral is found most frequently in nature as water-worn and apparently opaque 

 pebbles. 



It is easy by means of the polariscope to demonstrate the doubly refracting character 

 of transparent fragments of chrysoberyl. The double refraction of this mineral is not, 

 however, very strong, the two indices of refraction having been determined as 1'756 and 

 l"74i7 respectively. The strength of refraction possessed by chrysoberyl is thus also rather 

 small, and about the same as that of corundum. The refractive indices of chrysoberyl for 

 the various colours of the spectrum differ from each other but little, so that the dispersion 

 produced is slight and no prominent play of prismatic colours is seen. 



Chrysoberyl shows only a limited range of colour ; in Brazil, which is the most 

 important locality of the mineral, it varies from pale yellowish-green to golden yellow and 

 brownish-yellow. The Uralian crystals are of an intense green colour, varying in shade 

 from grass-green to emerald-green. There are thus two varieties of chrysoberyl to be 

 recognised, the one of a pale yellowish-green colour, considered as chrysoberyl proper, and 

 the darker emerald-green variety, which is distinguished by the name of alexandrite. The 

 first named is the more abundant and typical variety, the second being comparatively rare 

 and of less importance. 



CHRYSOBERYL PROPER AND CYMOPHANE. 



The common variety of chrysoberyl is typically pale in colour. Green strongly tinged 

 with yellow (Plate XII., Figs. 10 and 11), more or less bright olive-green, asparagus-green, 

 grass-green, and green inclining to grey or white, are all shades of colour of which examples 

 may be found among chrysoberyls. The more usual green colour sometimes merges into 

 o-olden-yellow, pale yellow, or brownish-yellow, occasionally even into brown or black. 

 Chrysoberyl is only feebly dichroic, the difference in the two colours of a crystal is not very 

 distinct even when it is examined with the dichroscope, and is barely perceptible to the 



