(IDOCRASE)
This
mineral affords transparent stones of pale brown, green, or yellow
colors, which closely resemble in appearance cut gems of smoky quartz,
tourmaline, chrysolite, hyacinth, or essonite. They have a rich luster
due to a combination of resinous and vitreous characters, and are
sufficiently dichroic to be of interest from that point of view. Nearly
all the cut stones come from the occurrence at Mount Vesuvius (whence
the mineral obtains its name), or from one on the Mussa Alp, in the Ala
valley of the Piedmont plateau, Italy. The crystals from Vesuvius are
generally brownish to colorless, while those of the Piedmont are green.
The
hardness of vesuvianite is 6.5, sufficient to give it a fair wearing
quality. Its specific gravity is 3.35 to 3.45. In composition it is a
complex silicate, chiefly of aluminum and calcium. It is fusible before
the blowpipe. It has a strong tendency to crystallize, the crystals
belonging to the tetragonal system, and usually appearing essentially
as short, stout prisms.
It
is not an uncommon mineral, but is usually too opaque to make desirable
gems. Its occurrence is especially associated with limestone, either as
the result of metamorphism or direct volcanic eruption, as at Vesuvius.
A yellowish brown variety, known as xanthite, occurs at Amity, New
York, and an occurrence on the Vilui River, near Lake Baikal, Siberia,
is known as viluite.
The
cut stones are made exclusively from clear crystals, which rarely
afford stones exceeding a few carats in size. The step or table cut is
the form usually given the stones.
