Iolite
Known also by various other names—cordierite, dichroite, and water-sapphire (saphire d’eau)—this species owes its interest to the remarkable dichroism characterizing it, the principal colours—smoky-blue and yellowish white—being in such contrast as to be obvious to the unaided eye. The stones that are usually worked have intrinsically a smoky-blue colour, and are found in waterworn masses in the river-gravels of Ceylon, whence is the origin of the name water-sapphire. Iolite, from ἴον, violet, and λίθος, stone, refers to the colour; cordierite is named after Cordier, a French geologist, who first studied the crystallography of the species; and dichroite, of course, alludes to the most prominent character of the species.
Iolite is a silicate of aluminium and of magnesium and iron corresponding to the formula H2(Mg,Fe)4Al8Si10O37. The double refraction is small in amount, biaxial in character, and negative in sign, the least and greatest of the refractive indices being 1·543 and 1·551; the specific gravity is 2·63, and hardness 7 on Mohs’s scale. Iolite, if used, is worked and polished; it is seldom faceted. A large worked piece, weighing 177 grams, which was formerly in the Hawkins Collection, is exhibited in the British Museum (Natural History).