Andalusite
Andalusite bears no resemblance whatever to kyanite, although, as has been stated above, the composition of the two species is essentially the same. It is usually light bottle-green in colour, and more rarely brown and reddish. Its extreme dichroism is its most remarkable character, the twin colours being olive-green and red. The reddish gleams that are reflected from the interior are in sharp contrast with the general colour of the stone, and impart to it a weird effect ([Plate XXIX], Fig. 15). Cut stones are often confused with tourmalines, and can, indeed, only be distinguished from the latter with certainty by noting on the refractometer the smaller amount of double refraction and the difference in its character. The least and greatest of the refractive indices are 1·62 and 1·643 respectively, and the double refraction, 0·011, about half that of tourmaline, is biaxial and negative; the specific gravity is 3·18, and hardness 7½ on Mohs’s scale.
Good stones are found at Minas Novas, Minas Geraes, Brazil, and in the gem-gravels of Ceylon. It was first known from the province of Andalusia, Spain, whence is the origin of its name.