Uraninite, or Pitchblende (10h), consists mainly of Uranium and Oxygen, but contains traces of Helium and Radium, of which latter it is the commercial source. Chrysoberyl (9e) is another precious stone, almost equal in lustre and hardness to the Sapphire; one variety is a beautiful greenish-yellow; another, with a peculiar play of light, is the Cat’s-eye; and a third, green by sunlight, but red by candle- or lamp-light, is known as Alexandrite. Corundum (9f), when clear and of the proper colour, is, after the Diamond, the most precious of stones. When pure, it is colourless, but with minute traces of colouring ingredient it assumes the richest and most varied hues; when red it is Ruby, and when blue Sapphire; the yellow, green, and purple varieties were at one time known respectively as the Oriental Topaz, Emerald, and Amethyst. The prefix “Oriental” was at first used to suggest that the stones are not ordinary Topaz, Emerald, and Amethyst, but other similarly coloured minerals coming from the East (India, Ceylon, Siam, Pegu, etc.); it was afterwards understood to suggest only the excellence of their characters. The Star-stone, another variety of Corundum, when placed in a strong light shows a six-rayed star. Hæmatite (11a) is a valuable ore of iron (Iron 70, Oxygen 30, per cent.). Cassiterite, or Tin-stone (11f), is the ore of Tin, of which metal it contains 79 per cent. Zircon (13b), when clear and without flaws, is one of the precious stones: one variety with peculiar red tints is the Hyacinth or Jacynth, while the colourless, yellowish, and dull green phases are termed Jargoon: the colourless variety, owing to its high refractive and dispersive power, approaches the Diamond in brilliancy. Quartz, which is Silica, the oxide of Silicon, is the most common of Minerals. In its clear and transparent variety it is the Crystal of the ancients and the Rock-Crystal of modern times; it is the Brazilian Pebble of spectacle-makers (14c). After the clear come the smoky varieties, including the Scotch Cairngorm and Occidental Topaz (14g). Next follows the Amethyst (14g), one of the less valuable, though one of the most beautiful, of gem stones. The Quartz Cat’s-eye (13f) is a variety presenting a similar play of light to that of the Chrysoberyl Cat’s-eye already referred to: the effect is due to enclosed fibres of an Asbestos-like mineral in the specimens from Ceylon, and to fibres of Crocidolite in the blue, and of altered Crocidolite in the brownish-yellow specimens from South Africa. Jasper (13g) is a coloured mixture of Silica and Clay, distinguished from ordinary Quartz by its opacity and dull earthy fracture. It is of various colours, chiefly red, brown, yellow, and green; the colours being arranged sometimes in a nodular form, as in Egyptian Jasper, at other times in stripes, as in Riband Jasper.