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.
