characteristic
of pearls and mother of pearls. A silky lustre is characteristic of
minerals with a fibrous structure. Lustre may be useful in the
determination of uncut material. Brilliancy is brought out by cutting
and polishing. The rough stone is very often a pebble, but its lustre
is brought out by cutting. Brilliancy also depends on transparency,
lustre and the fineness of the polish. A hard stone takes a better
polish and a stone with a greater refractive index possesses greater
lustre and brilliancy.
Transparency or Diaphaneity
A
substance is transparent when it transmits light through it, so
transparent minerals allow light to pass through them freely, e.g. rock
crystal. When light is only feebly transmitted through a mineral which
does not permit any object to be seen through it, it is called
translucent. The majority of gem minerals are transÂparent, and
particularly of the precious stones.
Fluorescence and Phosphorescence
Certain
minerals glow or become luminescent on heating or when subject to the
influence of ultraviolet rays, X-rays, Cathode rays, etc. It is well
observed in the dark and the luminescent colours differ from those of
unexcited minerals. At times these colours may be interesting and
spectacular. A substance is said to fluoresce, if the luminescence is
present only during the period of excitation, and a substance is said
to 'phosÂphoresce' if the luminescence persists even after removal of
the cause of excitation. The diamond, ruby, opal and amber show
luminescence to a marked extent. Both the above phenomena could be
demonstrated by the use of mercury vapour and argon lamps. At times
luminescence phenomena are useful in the identification of gem-stones.
Similarly, diamonds are transparent to X-rays, whereas glass imitations
are opaque.
18
2B
