162 A Book of Precious Stones
columnar and black, without cleavage or trace of fibrous texture.
Tourmaline
heated, like some other minerals in which one termination differs in
form from the other, develops electricity, with the effect of making of
the ends positive and negative poles. Sections of tourmaline crystals
cut parallel to the axis have the property of polarising light.
Tourmaline can be fused under the blowpipe to a spongy enamel; it melts
with borax to transparent glass. Tourmaline is cut step and brilliant.
Twin-coloured
tourmaline is strongly doubly refractive; green shows yellow and
greenish blue; yellowish green, yellow and green; reddish brown, light
and dark brown; red, pink and dark red; blue, light and dark blue. The
green tends toward blue while the blue has a greenish tendency. Some
brown tourmalines have mixed colours.
In
considering shades when selecting tourmalines, a medium bright green
is better than the lighter or that which appears blackish. The pink
should be deep and clean, ruby-like. A rich amber brown is most
desirable of the brown shades. Red tourmaline is occasionally so like
the ruby that it might deceive any but the ex-
