In its opaque form, colored either black or brown, Tourmaline is a comparatively common mineral. It accompanies many so-called metamorphic rocks, i. e., rocks which have been changed by heat and pressure from their original condition, and is also common in granite and other eruptive rocks. As a rock forming mineral it often occurs as long, slender prisms, frequently about the size of a darning needle and radiating in all directions. The only mineral for which it is likely to be mistaken in this form is Hornblende. It can be distinguished from this in the following manner: On fusing the powdered mineral with a mixture of bisulphate of potash and fluor spar (best done on a little loop of platinum wire) Tourmaline will color the flame green, while Hornblende will produce no coloration. The black opaque crystals often reach a large size, as some are known to be four feet in length. Both black and brown Tourmaline are usually opaque, and hence have no value as gems. The Tourmalines available for gems are transparent and have a great variety of color. The gem Tourmalines are to be found in only a few localities. They occur in Maine, Connecticut and California in our own country, and also in Brazil, Russia and Ceylon. The crystals are usually in the form of long, slender prisms. They often have the peculiarity of being differently colored in different portions. Thus a crystal may be green at one end and red at the other, and in cross section may show a blue center, then a colorless zone, then one of red and then one of green. Some of the crystals from Paris, Me., change from white at one termination to emerald green, then light green, then pink, and finally are colorless at the other termination. In some crystals again the red passes to blue, the blue to green and the green to black. [Illustration: TOURMALINE.] Left Column: Green Tourmaline (Brazil.) Green Tourmaline (Haddam, Conn.) Cross Section of Green Tourmaline (Cal.) Center Column: Red Tourmaline or Rubellite (Island of Elba.) Brown Tourmaline (Gouverneur, N. Y.) Red Tourmaline or Rubellite in Lepidolite (Cal.) Right column: Black Tourmaline (Finland).