It alloys with other metals, especially copper. The union of copper and a small amount of aluminum makes aluminum-bronze, which looks like gold and is used for watch chains, pencil-cases, etc., and also for the antifriction bearings of heavy machinery. A small amount added to steel prevents air holes and cracks in casting. Corundum Al₂O₃ Pl. 23 Occurs in cleavable masses or in hexagonal crystals; hardness 9; specific gravity 4; colorless, red, yellow, blue, or gray; luster vitreous to adamantine; translucent to transparent on thin edges. Corundum is readily recognized by its hardness, second only to that of the diamond. The crystals may be simple six-sided prisms, hexagonal pyramids or combinations of the two. The cleavage is usually described as parting, for it is by no means perfect, but when it is recognizable it is parallel to the faces of a rhombohedron, and cleavage pieces may appear almost cubic. When in clear and perfect crystals this mineral is one of the most highly prized of all the gems. Clear and colorless it is known as the “_Oriental white sapphire_”; when tinged with blue it is the _sapphire_; when colored yellow, the “_Oriental topaz_”; when green, the “_Oriental emerald_”; when purple, the “_Oriental amethyst_” and when red, the _ruby_. Sapphires range from colorless to deep blue, the value depending on the shade of the blue, and increasing as the color deepens. The Oriental topaz can easily be confused with the true topaz, which is a much commoner and less valuable gem, but can be distinguished by the hardness, topaz having a hardness of but 8. The name emerald is applied to several green gems, mostly to beryl, which is not so hard and is the true emerald. The Oriental emeralds have a value about the same as diamonds. Rubies of clear and deep color are the rarest of all gems, ranging in value about three times as high as diamonds of equal size. The most sought-for shade is the so-called “pigeon-blood red,” and the value of a stone of this sort is almost dependent on the whim of the buyer. The best of the rubies come from granites or metamorphosed limestones in Burma; the best sapphires from Ceylon, though both of these, and some of the other corundums of gem quality, have been found in North Carolina and Montana. Around these stones, which have been used so long among the Hindus, Persians, Jews, Egyptians, and Christians, a wealth of lore has been woven. The sapphire was Saturn’s stone, and a talisman to attract Divine favor. Where tradition makes the stone on which the ten commandments were written the sapphire, it is probable that, what was really meant, is lapis lazuli, as is also the case when sapphires are mentioned as building stones for the celestial gates. The ruby in ancient lore is termed “lord of stones,” “gem of gems” etc., and so protected its wearer that he was safe from injury in peace or war.