The species is interesting scientifically because it contains the rare element lithium; it is a silicate of aluminium and lithium, corresponding to the formula LiAl(SiO_{3})_{2}. The double refraction is biaxial in character and positive in sign, the least and greatest of the refractive indices being 1·660 and 1·675; the specific gravity is 3·185, and hardness 6½ to 7 on Mohs’s scale. Spodumene has an easy cleavage, and the cut stones call therefore for careful handling, lest they be flawed or fractured. Two faceted stones, a beautiful kunzite and a fine hiddenite, weighing 60 and 2½ carats respectively, are exhibited in the British Museum (Natural History). 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 H_{2}(Mg,Fe)_{4}Al_{8}Si_{10}O_{37}. 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). BENITOITE