DIOPSIDE
	This variety of pyroxene affords transparent green stones, which may resemble in color chrysolite or green tourmaline. Diopside is common as a rock-forming mineral, but is obviously of use for gem purposes only when occurring in large, transparent crystals. The three localities where material of the latter sort is chiefly obtained are the Ala Valley in the Piedmont region of Italy, the Zillerthal in the Tyrolese Alps, and De Kalb, St. Lawrence County, New York. The first and third localities afford light green stones, the second those of a dark bottle-green color. They are cut generally as brilliants, and while not extensively used, make satisfactory stones. Those obtained from De Kalb afford gems up to 10 carats in size. Diopside may be distinguished from gems of other min­erals of the same color by its lack of dichroism, this being a characteristic of this pyroxene. From glass it differs in being doubly refracting. Its system of crystallization is monoclinic. Its hardness is 6; specific gravity, 3-3.6. In composition diopside is a silicate of lime and mag­nesia, with a small amount of iron, its color growing darker with more iron. It has a prismatic cleavage, not often strongly enough developed, however, to interfere with cutting the mineral. Its luster is somewhat oily like that of chrysolite. The appearance of crystals from the Ala, accompanying essonite, is shown in the colored plate.
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