Chrysoberyl is in composition an aluminate of beryllium corresponding to the formula BeAl_{2}O_{4}, and is therefore closely akin to spinel. It usually contains some ferric and chromic oxides in place of alumina, and ferrous oxide in place of beryllia, and it is to these accessory constituents that its tints are due. Other gem-stones containing the uncommon element beryllium are phenakite and beryl. Pale yellowish green, the commonest colour, is supposed to be caused by ferrous oxide; such stones are known to jewellers as chrysolite (Plate XXVII, Fig. 12). Cat’s-eyes (Plate XXIX, Fig. 1) have often also a brownish shade of green. The bluish green and dark olive-green stones known as alexandrite (Plate XXVII, Figs. 11, 13) differ in appearance so markedly from their fairer sisters that their common parentage seems almost incredible. The dull fires that glow within them, and the curious change that comes over them at night, add a touch of mystery to these dark stones. Chromic oxide is held responsible for their colour. The cat’s-eyes are, of course, always cut _en cabochon_, but otherwise chrysoberyl is faceted. The name of the species is composed of two Greek words, χρυσός, golden, and βήρυλλος, beryl, and etymologically more correctly defines the lighter-coloured stones, which were, indeed, at one time the only kind known. Chrysolite from χρυσὁς, golden, and λίθος, stone, has much the same significance. This name is preferred by jewellers, but in science it is applied to an entirely different species, which is known in jewellery as peridot. Cymophane, from κῦμα, wave, and φαίνειν, appear, refers to the peculiar opalescence characteristic of cat’s-eyes; it is sometimes used to designate these stones, but does not find a place within the vocabulary of jewellery. Alexandrite is named after Alexander II, Czar of Russia, because it first came to light on his birthday. That circumstance, coupled with its display of the national colours, green and red, and its at one time restriction to the mining district near Ekaterinburg, renders it dear to the heart of all loyal Russians.