The name of the species comes through the French _corindon_ from an old Hindu word, _korund_, of unknown significance, and arose from the circumstance that the stones which first found their way to Europe came from India. At the present day the word corundum is applied in commerce to the opaque stones used for abrasive purposes, to distinguish the purer material from emery, which is corundum mixed with magnetite and other heavy stones of lower hardness. The origin of the word sapphire, which means blue, has been discussed in an earlier chapter (p. 110). Jewellers use it in a general sense for all corundum except ruby. Ruby comes from the Latin _ruber_, red. The prefix ‘oriental’ (p. 111) is often used to distinguish varieties of corundum, since it is the hardest of ordinary coloured stones and the finest gem-stones in early days reached Europe by way of the East. Corundum crystallizes either in six-sided prisms terminated by flat faces (Plate I, Fig. 10), which are often triangularly marked, or with twelve inclined faces, six above and six below, meeting in a girdle (Plate I, Fig. 14). Ruby favours the former and the other varieties the latter type. A fine crystal of ruby—the ‘Edwardes,’ so named by the donor, John Ruskin, after Sir Herbert Edwardes—which weighs 33·5 grams (163 carats), is exhibited in the Mineral Gallery of the British Museum (Natural History), and is tilted in such a way that the light from a neighbouring window falls on the large basal face, and reveals the interesting markings that nature has engraved on it. From its type of symmetry corundum is doubly refractive with a direction of single refraction running parallel to the edge of the prism. Owing to the relative purity of the chemical composition the refractive indices are very constant; the ordinary index ranges from 1·766 to 1·774 and the extraordinary index from 1·757 to 1·765, the double refraction remaining always the same, 0·009. The amount of colour-dispersion is small, and therefore colourless corundum displays very little ‘fire.’ The difference between the indices for red and blue light is, however, sufficiently great that the base of a ruby may be left relatively thicker than that of a sapphire to secure an equally satisfactory effect (cf. p. 98)—a point of some importance to the lapidary, since stones are sold by weight and it is his object to keep the weight as great as possible. When a corundum is tested on the refractometer in white light a wide spectrum deliminates the two portions of the field because of the smallness of the colour-dispersion (cf p. 25). The dichroism of both ruby and sapphire is marked, the twin colours given by the former being red and purplish-red, and by the latter blue and yellowish-blue, the second colour in each instance corresponding to the extraordinary ray. Tests with the dichroscope easily separate ruby and sapphire from any other red or blue stone. This character has an