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Ch. 1: General Descriptions of Precious Stones

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16
PRECIOUS STONES
displacement of five carats, and the result, 3, would be the specific gravity of the stone. Thus, absolute weight, fifteen carats; specific weight, ten carats; loss, five carats: 15 -5-5 = 3, specific gravity.
All minerals are electric. Some acquire electricity by heating, some by friction, others by either method. Some display positive, and others negative, electricity. Although the diamond is a non-conductor, it becomes positively electric by friction, and it differs from other stones in that it is elec­tric in the rough also. And the different stones vary in their power to retain electricity: some can do so for a few minutes only; others for many hours.
Those minerals which become electric by heating are said to be pyro-electric, from the Greek nop, fire, and electric. The tourmaline, when heated, shows positive electricity at one end of the prism, and negative at the other. The other hemihedrally modified prisms (topaz, for instance) have the same peculiarity.
The chrysolite and several forms of garnet possess the power to act on the magnetic needle. The tourmaline and others, after friction, will attract and hold small bits of paper and the like; and the Brazilian topaz, made electric by fric­tion or heating, will affect the electric needle after many hours.
Many precious stones become distinctly phosphorescent by exposure to sunlight, or by the application of heat, or other electrical and mechanical methods. This is true of the dia­mond, rock crystal, ruby, zircon, and fluorite. Some dia­monds are much more phosphorescent than others. There are diamonds which assume a deep violet hue under the arc light. Upon experimenting with stones of this character, Mr. George F. Kunz found that they displayed phosphores­cence to an unusual degree, possessing the quality of storing up sunlight and electric light, and emitting the same in the dark. He also observes that these stones are generally slightly
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