Gems Notes and Extracts Augusto Castellani, Mrs. John Brogden 1871

INTRODUCTION. 9Many crystals present beautiful changes of light,when this is reflected in certain given directions, asin the Labradors, the moon-stone, and the Asteria.Minute crevices cause the iridescence, as in the stonecalled Iris.Many gems are more or less transparent ; that is,they possess in a greater or less degree the power oftransmitting light; and, according to these degrees,they have different denominations. That stone iscalled " transparent " which, when placed before anobject, does not impedeits view.When the stone allows an object to be seen indistinctlythrough it, it is called " semi-transparent," and" translucent " when it allows the light to pass, buthinders the view of objects.ELECTRICITY.Electricity is produced in gems by heat, friction, orviolence. In some gems it is caused by warming themat the fire,and then they are said to possess pyrelectricism.Electricity is produced by friction in the topaz,the amethyst, all quartz, the diamond, the garnet andemerald. Some crystals become electrical by percussion,as, for instance, the Iceland spar.Some stones conduct, others rejectelectricity, whichin those cases is either positive or negative.The power of retaining electricity when producedis very variable in stones, and the time which elapses