Silica 40.
Alumina 39.16
Lithia and potash 3.59
Protoxide of iron 5.96
Protoxide of manganese 2.14
Boracic acid 4.59
Volatile matter 1.58
The tourmaline possesses double refraction to a high degree, and its
power of polarizing light is so great that, cut into slices, it is used
in the polariscope for analyzing other minerals.
If two slices of tourmaline, cut parallel to their axis, be laid one on
the other in one direction, both are transparent; if laid in another
direction they become opaque, and if a doubly refracting crystal be
placed between the two plates of tourmaline, the part covered by the
crystal is transparent while the other is opaque.
Tourmaline melts with borax into a transparent glass; the rubellite
turns white, and the indicolite and green tourmalines turn black, under
the blow-pipe.
Tourmalines can be distinguished from other gems by their specific
gravity, but principally by their property of assuming polaric
electricity after being heated, one end becoming positive and the other
negative.
The history of the discovery of the tourmaline and its electric
property is a curious one.
On a warm summer day, early in the eighteenth century, some children
were playing in a courtyard in Amsterdam. Amongst their playthings
were some precious stones which the Dutch navigators had brought from
Ceylon. Some of the stones seemed to be possessed of the strange power
of attracting and repelling small bits of straw, ashes, and other light
substances. The little ones called their parents to witness this
strange phenomenon, and the stolid Dutch lapidaries, themselves puzzled
at the sight, called the stones _aschentreckers_ or ash-drawers.
A number of years afterwards, careful experiments disclosed the
wonderful electric powers of the aschentreckers or tourmalines. Purple,
green, and blue tourmalines are found in Brazil. In Ceylon the stones
are found in gravel beds. Rubellites or siberites are found in Siberia.
Tourmalines are also found in Moravia, the island of Elba, Sweden,
Burmah, Tyrol, Canada, and the United States.
The first tourmaline deposits known in the United States were
discovered at Paris, Maine, in 1820. Another wonderful deposit was
found at Mt. Apatite in Maine in 1882, and up to the present time the
finest tourmaline crystals have been discovered in the United States.
Really fine specimens of red, blue, or green tourmalines are uncommon
and command very good prices.