colour
in the tourmaline is a wide one, ar.d when we take into consideration
that these differences of colour also denote differences of
composition, we see how interesting these precious stones are from a
scientific point of view.
The
phenomena also exhibited by the tourmaline are remarkable, the most
important being its power of polarizing light, and it has rendered the
greatest assistance to physicists in their study of the properties and
laws governing light. The discovery by Huyghens in the year 1678 of the
polarization of light by double refraction, as exemplified by Iceland
spar, paved the way for experiments with the tourmaline, when this
property had been discovered in that gem. The discovery led to the use
of the tourmaline in most subsequent experiments made with polarized
light. The history of these investigations is most interesting and
scientifically useful, but foreign to our present purpose.
Pyroelectricity, or
that property certain substances have of acquiring electricity when
heated, is best studied in the tourmaline ; in fact it was first
discovered in this mineral. Its power of attracting and then repelling
hot ashes when placed among them, directed attention to this
peculiarity of the tourmaline, and was on this account called by the
Dutch the ashes-drawer. For further information concerning this
phenomenon of pyroelectricity works on physics may be consulted, but
the electrical properties of tourmaline when heated, may he determined
by carefully heating the specimen in the flame of a candle, when minute
pieces of paper, pith, or other light substances become attracted in a
most marked manner.
It
is generally supposed that the first tourmaline was brought from
Ceylon, and the name is no doubt of Cingalese origin, but it appears to
have been little thought of until the beginning of the eighteenth
century, an account of it appearing in the Memoirs of the Academy of
Sciences at Paris for the year 1717 ; so that in the list of precious
stones the tourmaline may be considered to have been brought into use
in comparatively recent times.
The
powerful dichroism of this gem acts in a remarkable manner, and alone
helps to identify it. Great care has to be taken in the cutting and
polishing of these stones, to prevent this dichroism acting in an
unsatisfactory manner. For instance, in cut and faceted specimens if
the table is perpendicular to the principal axis of the crystal, most
probably the gem will appear in its thicker portion opaque and black,
but by making the table parallel with this axis the gem will present a
brilliant and beautiful appearance ; many of the green stones, or Brazilian Emeralds, vieing
with the emerald for beauty of colour and appearance, having a fine
play of colours, and often exhibiting two hues of green in the same
stone. Even naturally the hues are not constant, and crystals have been
found, one part of which are green, another part red. Some are crimson
tipped with black, others yellowish mixed with carmine, and some are
blue passing into green, while some very fine specimens found in
America are red internally and green on the outside. In the Isle of
Elba specimens are found which are red at one end, yellow in the
middle, and black or brown at the other extremity.
The
colourless tourmalines are very rare. They are found in Siberia and
Elba, the best come from the latter locality, but are not usually
sufficiently perfect or of sufficient size to be cut into gems.
Ceylon
and India furnish a large quantity of these gem-stones, and a
magnificent group of crystals, presented to Colonel Symes in 1799 by
the King of Burmah, and now in the British Museum, has been valued at
£1,000. It is the variety rubellite, or red tourmaline. That
institution contains a splendid collection of tourmalines from nearly
every locality where found—a specimen from the Tyrol showing both ends
of the crystal
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