Spinel.—A mineral occurring in octahedrons of great hardness and various colors, as red, green, blue, brown, and black, the red variety being the gem spinel ruby. It consists essentially of aluminum magnesium, but commonly contains iron and sometimes also chromium. The fine specimens of spinel ruby are worth rather more than half as much as the diamond.
Topaz.—A mineral occurring in rhombic prisms, generally yellowish and pellucid, also colorless, and of greenish, bluish, or brownish shades. It sometimes occurs massive and opaque.
Tourmaline.—A mineral occurring in three-sided prisms. Black tourmaline is the most common variety, but there are also other varieties, as the blue (indicolite), red (rubellite); also green, brown, and white. The red and green varieties, when transparent, are valued as jewels. The finest ones come from Maine, and are worth four or five times as much as garnets.
Turquoise.—A hydrous phosphate of alumina containing a little copper. It has a blue, or bluish-green color, and usually occurs in kidney-shaped masses with a nodular surface like that of a bunch of grapes. The finest specimens are worth nearly half as much as diamonds.
Verd antique.—A mottled-green, serpentine marble, also a green porphyry, which is called oriental verd antique.
Zircon.—A mineral usually of a brown or gray color. It consists of silicon and zirconium, and is harder than the garnet. The transparent varieties are used as gems. The red variety is called Hyacinth; a colorless, pale yellow, or smoky-brown variety from Ceylon is called jargon.
Gold, a metal valued on account of its scarcity, color, luster, and power of resisting oxidation. It is found in nearly all parts of the world. South Africa and the United States are the leading producers. Australia, South America and parts of Europe possess important gold fields.
Gold is separated from gravel (placer mines) by washing with water. The particles of metal, being heavy, sink and can be collected. Rock containing gold is crushed to fine powder and the gold combined with mercury (amalgamation). Low-grade ores are treated with a solution of cyanide of potassium which dissolves the gold and the metal is later separated.
Chloride of gold, used in photographic work, is its only important compound. Pure gold is called twenty-four carats fine. A smaller figure indicates that the metal is alloyed to harden it.
Gold is used for money, jewelry, gold leaf (gilding) and in dentistry. It is almost always alloyed with copper and silver. Gold is the world’s accepted standard of value. Shipments of gold go from one country to another chiefly to balance international business dealings. Government treasuries and bank vaults [113] are the chief storehouses for gold, either as bullion or coin.