Chrysoberyl crystallizes in the orthorhombic system, and occurs in rather dull, complex crystals, which are sometimes so remarkably twinned, especially in the variety called alexandrite, as to simulate hexagonal crystals. In keeping with the crystalline symmetry it is doubly refractive and biaxial, having two directions of single refraction. The least and the greatest of the principal indices of refraction may have any values between 1·742 and 1·749, and 1·750 and 1·757, respectively, the maximum amount of double refraction remaining always the same, namely, 0·009. The mean principal refractive index is close to the least; the sign of the double refraction is therefore positive, and the shadow-edge corresponding to the lower index, as seen in the refractometer, has little, if any, perceptible motion when the stone is rotated. The converse is the case with corundum; the sign is negative, and it is the shadow-edge corresponding to the greater refractive index that remains unaltered in position on rotation of the stone. This test would suffice to separate chrysoberyl from yellow corundum, even if the refractive indices of the former were not sensibly lower than those of the latter. Also, the dichroism of chrysolite is stronger than that of yellow sapphires. In alexandrite this phenomenon is most prominent; the absorptive tints, columbine-red, orange, and emerald-green, corresponding to the three principal optical directions, are in striking contrast, and the first differs so much from the intrinsic colour of the stone as to be obvious to the unaided eye, and is the cause of the red tints visible in a cut stone. The curious change in colour of alexandrite, from leaf-green to raspberry-red, that takes place when the stone is seen by artificial light, is due to a different cause, as has been pointed out above (p. 54). The effect is illustrated by Figs. 11, 13 on Plate XXVII, which represent a fine Ceylon stone as seen by daylight and artificial light; the influence of dichroism may be noticed in the former picture. The specific gravity of chrysoberyl varies from 3·68 to 3·78. In hardness this species ranks above spinel and comes next to corundum, being given the symbol 8½ on Mohs’s scale. Certain stones contain a multitude of microscopic channels arranged in parallel position. When the stones are cut with their rounded surface parallel to the channels, a broadish band of light is visible running across the stone at right angles to them, and suggests the pupil of a cat’s eye, whence the common name for the stones. The fact that the channels are hollow causes an opalescence, which is absent from the quartz cat’s-eye.