Diamonds and precious stones, a popular account of gems .. . x ; there are, how-ever, certain elements which are characteristic of it,namely, boracic acid, silica, and alumina. In alltourmalines, also, there is an alkaline base, some-times potash, sometimes soda, sometimes lithia, ora mixture of all. There is found in it also mag-nesia, lime, oxide of iron, and oxide of manganese. The tourmaline is always crystallized, and itscrystals appertain to the rhombohedral crystals are in the form of longish prisms,sometimes with six faces, sometimes with nine,and in this case, in consequenc
Diamonds and precious stones, a popular account of gems .. . x ; there are, how-ever, certain elements which are characteristic of it,namely, boracic acid, silica, and alumina. In alltourmalines, also, there is an alkaline base, some-times potash, sometimes soda, sometimes lithia, ora mixture of all. There is found in it also mag-nesia, lime, oxide of iron, and oxide of manganese. The tourmaline is always crystallized, and itscrystals appertain to the rhombohedral crystals are in the form of longish prisms,sometimes with six faces, sometimes with nine,and in this case, in consequence of the obliterationor partial obliteration of faces, they terminate insuch forms as shown in Fig. 82. LABRADORITE. l8l The tourmaline assumes a great many colours,and accordingly receives a great number of Isle of Elba produces specimens from whiteto black ; a species from Siberia is a beautiful red ;from the Brazils both green and blue tourmalinesare brought; and from Sweden a fine indigo-bluecalled indicolite. Berlin-blue tourmaline is called. Fig. 82.—Tourmaline. Brazilian sapphire ; and there are green shades,which are called Ceylon or Brazilian emeralds. These blue and green tourmalines show in themost pronounced manner the phenomenon of di-chroism. LABRADORITE. The beautiful stone of Labrador, unknown until, 182 PRECIOUS STONES. about a hundred years ago, it was discovered in theIsland of St. Paul, near the coast of Labrador, isthe type of one of the four great species formed byRose, at the expense of the ancient group of thefelspars. Its specific gravity is 26 to 27. The labradorite is called sometimes the opalinefelspar, because it resembles the opal in its beauti-ful reflections of colours. The finest colours area play of blue and green upon gray. Yellow andbronze-red are less common ; and a pearly-gray andpurple-red still more rare. The labradorite is essentially formed of silica,alumina, lime, and soda. It has been found in its crystallized statescatter
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