PRECIOUS STONES. 215
translucent,
of specific gravity 2"63 to 2'67. Its hardness is rather greater than
the previous species, being 6—7. The cleavages are very similar to
those of other triclinic Felspars. It occurs in well developed crystals
of the triclinic system, and in crystalline masses. In the Aventurine
variety (Heliolite) the inclusion is usually Haematite, possibly
sometimes Gothite, but it always occurs so as to give the sheen
parallel to the principal cleavage plane; on this aspect, too, it shows
the characteristic striping due to polysynthetic twinning common to all
triclinic Felspars. This striping affords a means of distinguishing
Oligoclase Sunstone from the artificial glass and from the Aventurine
Quartz; from the glass it may also be distinguished by its double
refraction. Quartz is usually distinctly harder also than Oligoclase.
It was at first only known from Cedlovatoi, an island near Archangel.
At that time—early in the nineteenth century—it was in great demand,
and brought a high price. Later it was found near Lake Baikal, and at
Tvedestrand in Norway. It also occurs in County Donegal, and in the
island of Tiree in Scotland. There are several localities where it is
found in iSlorth America. It is cut en cabochon, so as to expose the metallic reflection to the observer.
319. Labradorite.
This
Felspar is rarely seen in distinct crystals, more often in large
crystalline masses of a dull grey or brownish colour, very rarely
colourless ; the general lustre is vitreous; very rarely is it seen
transparent, usually being only sub-translucent, or even opaque. It
has two cleavages inclined to one another at a little over a right
angle, and, in addition,
