The natural crystals have the form of a six-sided prism, and in the case of emerald (Fig. 70, and Plate I, Fig. 8) invariably, if whole, end in a single face at right angles to the length of the prism; aquamarines have in addition a number of small inclined faces, and stones from both Russia and Brazil often taper owing to the effects of corrosion. The sixfold character of the crystalline symmetry necessarily entails that the double refraction, which is small in amount, 0·006, is uniaxial in character, and, since the ordinary is greater than the extraordinary refractive index, it is negative in sign. The values of the indices range between 1·567 and 1·590, and 1·572 and 1·598 respectively, in the two cases, the pink beryl possessing the highest values. The dichroism is distinct in the South American emerald, the twin colours being yellowish and bluish green, but otherwise is rather faint. The specific gravity varies between 2·69 and 2·79, and is therefore a little higher than that of quartz. If, therefore, a beryl and a quartz be floating together in a tube containing a suitable heavy liquid, the former will always be at a sensibly lower level (cf. Fig. 32). The hardness varies from 7½ to 8, emerald being a little softer than the other varieties. There is no cleavage, but like most gem-stones beryl is very brittle, and can easily be fractured. Stones rendered cloudy by fissures are termed ‘mossy.’ When heated before the blowpipe beryl is fusible with difficulty; it resists the attack of hydrofluoric acid as well as of ordinary acids.