shades
of Tourmaline are particularly dichroic. The electrical properties are
equally well marked ; thus rubbing induces a difference of electric
potential, and heat produces a marked difference, one end of the
crystal becoming positive and the other negative, so that if a mixture
of red lead and sulphur be dropped on to a heated crystal through a
fine sieve the sulphur will be attracted to one end (the positive) and
the red lead to the other (the negative) ; this readily distinguishes
the two ends ; the electrical properties are in some cases of value in
identifying Tourmaline. The specific gravity varies from 2"98 to 3'20,
but most of the gem varieties have a nearly constant density, as will
he noted below. The mineral is brittle and has a subconchoidal
fracture. Its hardness is 7 to 7-1/2. There are two indefinite
cleavages ; the streak is colourless. In crystalline form it is
rhombohedral and is hemimorphic, but as doubly terminated crystals are
rare, this is not often seen in the ends, but in the prism faces it is
usually apparent, for the crystal in section, instead of being
hexagonal, has the form of a modified triangle through the predominance
of alternate faces of the hexagonal prism (Fig. 39). Another peculiar
feature of the crystals which throws light on the mode of origin of the
mineral is the frequency with which the prism faces are well developed
in comparison with the terminal planes. The crystals, too, are very
frequently fractured and dislocated. It is a mineral of secondary
origin usually found in association
÷ 2
