|
PRECIOUS STONES. 279
continuity
with it; this may be compared with the well-known double growths of
common alum and chrome-alum. Eubellite is apt to appear of a brown
colour in artificial light. Coloured Tourmaline is usually cut in steps
or as a table, and the appearance is often improved by the gem being
set in a closed mount with foil.
Tourmaline,
although in its various colours it so closely resembles in outward
appearance so many other gems, may in most cases be identified by its
specific gravity alone. Thus Aquamarine, Emerald, and Beryl generally,
Quartz and Phenakite are all less dense. Hiddenite is slightly more
dense, while Corundum in all its varieties, Garnet in its different
forms, Olivine, Topaz, Spinel, Zircon, and Diamond are all of
distinctly greater specific gravity. Further, the green and blue kinds
of Tourmaline are very markedly dichroic, and this aids in the further
differentiation of the green Hiddenite. The similarity of the coloured
images seen in Tourmaline under the dichroscope, and its marked
electrical properties are additional points of distinction.
428. Staurolite.
Staurolite
suitable for gem use occurs but sparingly, and it is a mineral but
little used for this purpose. What is so used is a reddish-brown,
sometimes inclining to claret, and is transparent, though the ordinary
forms are only translucent to opaque. The mineral shows double
refraction (the greatest and least indices being 1.746 and 1.736) and
is distinctly pleochroic, being hyacinth-red in one direction and
yellow-red in the other two directions. The specific gravity is 3'73 to
3'75, and the hardness 7-1/2. The
|
||
|
|
||