If two slices of tourmaline, cut parallel to their axis, be laid one on the other in one direction, both are transparent; if laid in another direction they become opaque, and if a doubly refracting crystal be placed between the two plates of tourmaline, the part covered by the crystal is transparent while the other is opaque. Tourmaline melts with borax into a transparent glass; the rubellite turns white, and the indicolite and green tourmalines turn black, under the blow-pipe. Tourmalines can be distinguished from other gems by their specific gravity, but principally by their property of assuming polaric electricity after being heated, one end becoming positive and the other negative. The history of the discovery of the tourmaline and its electric property is a curious one. On a warm summer day, early in the eighteenth century, some children were playing in a courtyard in Amsterdam. Amongst their playthings were some precious stones which the Dutch navigators had brought from Ceylon. Some of the stones seemed to be possessed of the strange power of attracting and repelling small bits of straw, ashes, and other light substances. The little ones called their parents to witness this strange phenomenon, and the stolid Dutch lapidaries, themselves puzzled at the sight, called the stones _aschentreckers_ or ash-drawers. A number of years afterwards, careful experiments disclosed the wonderful electric powers of the aschentreckers or tourmalines. Purple, green, and blue tourmalines are found in Brazil. In Ceylon the stones are found in gravel beds. Rubellites or siberites are found in Siberia. Tourmalines are also found in Moravia, the island of Elba, Sweden, Burmah, Tyrol, Canada, and the United States. The first tourmaline deposits known in the United States were discovered at Paris, Maine, in 1820. Another wonderful deposit was found at Mt. Apatite in Maine in 1882, and up to the present time the finest tourmaline crystals have been discovered in the United States. Really fine specimens of red, blue, or green tourmalines are uncommon and command very good prices. OPAL. The precious or noble opal, fire opal, common opal, hydrophane, and cachelong are different varieties of a mineral that is composed of about nine parts silica and one part water. The colors vary from chalky-white to bluish-white, from yellow to red, and from a slight play of colors to the beautiful mingling of green, blue, and red with the most remarkable kaleidoscopic effects. The opal is 5.5 to 6 in hardness, specific gravity 2 to 2.1, lustre glassy, and translucent from a slight to a very high degree. The opal is found in an amorphous state and never crystallizes; in fact from the condition of the pockets in which this mineral is found, the indications are that the substance was once a fluid.