Three varieties of chrysoberyls are known as cat’s-eyes, cymophanes, and alexandrites.
The chrysoberyl crystallizes in the trimetric or rhombic system; the cleavage is imperfect; fracture conchoidal; hardness, 8.5, being the third hardest stone; specific gravity, 3.65 to 3.8; and lustre vitreous to greasy.
The composition of the chrysoberyl is: alumina, 80.2; glucina, 19.8; with traces of protoxide of iron and oxides of lead and copper. The chrysoberyl is doubly refractive to a high degree, acquires positive electricity lasting several hours, is infusible alone, but melts with borax or salts of phosphorus to a clear glass, though with difficulty.
The chrysoberyl is unaffected by acids, but with a solution of cobalt nitrate the powdered mineral becomes blue.
Transparent greenish-yellow chrysoberyls are sometimes called Oriental chrysolites. These, and the brownish-yellow stones are the gems most used in jewelry.
The chrysoberyl cat’s-eye, or Ceylon cat’s-eye, is found in various shadings of yellow, brown, and green, and sometimes nearly black. These stones are translucent to opaque, and have a bright band of light running through the centre. This band is nearly always white, and in fine specimens is sharply defined, not too wide, and is in the centre of the stone.
The cat’s-eye chrysoberyls are always cut convex or cabochon shape, and as the stone is moved from side to side the band of light moves over its surface.
Cymophane.
The cymophane, or floating light, as the name denotes, is a chrysoberyl with a bright spot of light which seems to float over the surface as the stone is moved. The cymophane is also cut cabochon.