This variety of felspar is called orthoclase, adularia, and orthose,
besides the commercial names of fish-eye, Ceylon or water opal, and in
the yellow and red tints sunstone. Moonstone occurs in crystals and
crystalline fragments, also massive and granular; its hardness is 6. to
6.5, specific gravity 2.4 to 2.6, refraction double, is not attacked by
acids, and is composed of:
Silica 64.5
Alumina 18.5
Potash 17.
With traces of soda.
This beautiful stone is the clearest of all varieties of felspar. It
is colorless, or only slightly tinted with blue, green, yellow, and
flesh-red, and is transparent to translucent.
The lustre is vitreous, and a brilliant pearly streak of white light
plays from side to side.
The yellowish- and reddish-tinted specimens are called sunstones,
and are quite rare. These sunstones must not be confounded with the
avanturine or felspar sunstone.
Moonstones are found principally in Ceylon and on the St. Gothard in
Switzerland, but also occur in Bavaria, Greenland, Tyrol, Dauphine,
Norway, and the United States.
During the past few years, large quantities of moonstone balls, cut
like whole pearls, have been used for jewelry—the stones being much
sought as well because of their beauty as on the ground of the popular
superstition that they will bring good luck to the wearer.
Small pieces or balls are not very valuable, but large perfect
specimens command a good price.