In composition corundum is alumina, oxide of aluminium, corresponding to the formula Al_{2}O_{3}, but it usually contains in addition small quantities, rarely more than 1 per cent., of ferric oxide, chromic oxide, and perhaps other metallic oxides. When pure, it is colourless; the splendid tints which are its glory have their origin in the minute traces of the other oxides present. No doubt chromic oxide is the cause of the ruddy hue of ruby, since it is possible, as explained above (p. 117), closely to imitate the ruby tint by this means, but nothing approaching so large a percentage as 2½ has been detected in a natural stone. The blue colour of sapphire may be due to titanic oxide, and ferric oxide may be responsible for the yellow hue of the ‘oriental topaz,’ as the yellow corundum is termed. Sapphires, when of considerable size, are rarely uniform in tint throughout the stone. Alternations of blue and red zones, giving rise to an apparent purple or violet tint, and the conjunction of patches of blue and yellow are common. Perfectly colourless stones are less common, a slight bluish tinge being usually noticeable, but they are not in much demand because, on account of their lack of ‘fire,’ they are of little interest when cut. The tint of the red stones varies considerably in depth; jewellers term them, when pale, pink sapphires, but, of course, no sharp distinction can be drawn between them and rubies. The most highly prized tint is the so-called pigeon’s blood, a shade of red slightly inclined to purple. The prices for ruby of good colour run from about 25s. a carat for small stones to between £60 and £80 a carat for large stones, and still higher for exceptional rubies. The taste in sapphires has changed of recent times. Formerly the deep blue was most in demand, but now the lighter shade, that resembling the colour of corn-flower, is preferred, because it retains a good colour in artificial light. Large sapphires are more plentiful than large rubies, and prices run lower; even for large perfect stones the rate does not exceed £30 a carat. Large and uniform ‘oriental topazes’ are comparatively common, and realize moderate prices, about 2s. to 30s. a carat according to quality and size. Green sapphires are abundant from Australia, but their tint, a kind of deep sage-green, is not very pleasing. Brown stones with a silkiness of structure are also known.