This
mineral affords a semi-transparent stone, which, when of a deep
oil-green color, may have a limited use in jewelry. It does not often
occur in nature in the form of large distinct crystals, but usually as
aggregates of minute crystals, in firm incrusting masses, with a
radiated structure. Portions of these masses, when of a uniform color,
form, when cut en cabochon, pleasing stones.
In
composition prehnite is a hydrous silicate of aluminum and calcium,
having the percentages: silica 43.7, alumina 24.8. It is easily fusible
before the blowpipe and is attacked by acids. Its hardness is 6.65;
specific gravity 2.80-2.95; its luster is vitreous. Its occurrence is
almost wholly in connection with basic eruptive rocks, in the veins and
cavities of which it forms a secondary mineral.
Some
of the most richly colored prehnite known is obtained at Pater-son and
Bergen Hill, New Jersey. In the Lake Superior region prehnite
accompanies native copper, and affords a stone which is considered
worthy of cutting. Many localities in the Alps furnish prehnite, and
handsome pieces are obtained by polishing masses occurring in China.
Some prehnite comes from the Cape of Good Hope, and when cut is known
as "Cape chrysolite."
