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are amenable to such treatment, e.g. yellow topaz becomes rosy pink on
heating, amethyst becomes yellow on heating and is sold as topaz.
Diamond when exposed to radium becomes green. Size, transparency and
glossiness are also important factors that create distinction in gems,
e.g. a five carat and a ten carat stone of aquamarine do not show the
same colour. In turquois the lack of transparency gives some
characteristic colours. Glossiness is more appreciated in opaque
material.
Gem-stones under Ultraviolet Light
When
a ruby or red spinel is exposed to ultra-violet rays in a dark room the
stones glow with a wonderful red light. Similarly, when a handful of
diamonds is held in front of an ultraviolet lamp, some shine with a
cornflower blue, some with a soft yellowish or green. This is called
fluoresr cence as it was first seen in fluorite.
Play
of colours is another feature as revealed by opal, due to a variety of
internal hues, and also by labradorite. The cause of this phenomenon is
the interference of light caused by minute cracks or inclusions within
the mineral. This is different from 'fire'. Opalescence is shown by
opal and moonstone and is due to milky or pearly reflections from the
interior of the crystal.
LUSTRE AND BRILLIANCY
The surface appearance of a mineral in reflected light is its lustre, and is governed by transparency, refractivity and structure
of the mineral. There are two kinds of lustre, metallic and
non-metallic. We are concerned with non-metallic lustre only for the
gem-stones, and the following varieties may be noted.
Adamantine is typical of diamond and other minerals
with high refractive index. Vitreous or glassy is possessed
by quartz and glass. Resinous is the lustre of resin, and
greasy denotes that of an oily surface. Pearly lustre is
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