The cymophane, or true cat's-eye, owes its chatoyancy, whether of pale steely whiteness as a flash, or as a line like a silver wire, to the orderly arrangement of an immense number of minute cavities along certain lines causing minute internal striations. The dark yellowish green hue is most prized; it is usually cut en cabochon. The chrysoberyl occurs in many localities, notably in Brazil and Ceylon, Connecticut, and the Urals. A fine specimen from the Hope collection is in the British Museum.
The chrysoberyl owes its colour chiefly to iron in the form of ferrous oxide; but traces of chromium and of manganese oxide also occur in it. Its percentage composition is roughly:
Alumina . . . . 78 | Glucina . . . .18 Ferrous oxide . . .4
Phenakite.
Phenakite is but rarely used as a gem-stone. The colourless transparent variety may, however, be mistaken for a diamond, especially by candlelight, when the prismatic colours, or "fire," of a brilliant-cut specimen are conspicuous. The hardness of this stone lies between 7½ and 8, while its specific gravity is close upon 3. Crystals of phenakite usually take the form of a low obtuse rhombohedron. This mineral is sometimes perfectly colourless and transparent, but more frequently is rather clouded and milky, or of a straw, sherry, or cinnamon tint. When viewed with a dichroiscope the ordinary image is colourless, the extraordinary image being of a warm yellow or brown, should the specimen examined possess any colour at all.
The best specimens of phenakite known come from the emerald and chrysoberyl mines at Takovaya, eighty-five versts east of Ekaterinburg, Perm, Asiatic Russia: the matrix is a mica-schist; less important examples are found in Colorado, U.S.A.
In the Mineralogical Gallery of the British Natural History Museum, there are fine specimens of phenakite both in crystals
