IV. The next division consists of compounds of Oxygen and includes most of the stony Minerals.
Cuprite (10a), an important ore of Copper (Copper 89, Oxygen 11, per cent.), is at first ruby-red in colour, but becomes blackened by exposure to light.
Spinel (10e), in its transparent varieties, is one of the precious stones; the deep red being the Spinel Ruby (less dense and less hard than the true Ruby), the rose-tinted the Balas Ruby, and the yellow or orange-red the Rubicelle of the jewellers: sometimes, it has a dark blue colour. On account of their hardness, the less valuable specimens are used for the jewelling of watches.
Magnetite, or Magnetic Iron-ore (10f), the richest ore of Iron, of which it contains 72 per cent., is the natural magnet.
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.