unmounted
gem and tests should be made on the girdle of the stone. A light
scratch on the girdle can be easily concealed by the setting of the
stone. Hardness of the stones could also be tested by the use of a file
or knifepoint (H = 6-1/2), and by trying it on a sheet of glass (H =
5-1/2). By this means a glass imitation could be easily tested. The
harder the stone, the easier it scratches glass. Hardness determination
should be done with caution, otherwise a good stone may easily be
spoiled. Hardness may also vary on different faces in crystals, e.g.
kyanite, where the direction parallel to the long axis is softer than
the one perpendicular to it.
Harder
minerals scratch or abrade softer ones, so good mineral specimens or
gems should not be jumbled together in a box lest the softer may
suffer, e.g. amber and pearls. The hardness of a mineral also
determines its durability. A gem should at least be harder than quartz
to retain its beauty, brilliancy and splendour. The diamond, ruby,
sapphire and emerald are harder than quartz. The softer stones are
often considered semi-precious (see Table of Hardness for Gem-stones,
pp. 166-167).
Cleavage
This
is related to the internal structure of crystals and forms an important
character. It is the property of easily splitting or separating along
definite planes, known as the cleavage planes, e.g. mica which has a
perfect basal cleavage. Crystals of different minerals have different
degrees of cleavage, which also serve in determining minerals. The
different degrees are known as 'perfect', 'imperfect', 'distinct' or
'indistinct'. Cleavage is also a property usually taken advantage of in
cutting gem-stones, e.g. diamond. Gem-stones with easy cleavage should
be handled carefully while being cut or polished. Cleavage is not
related to hardness and is possessed by both soft and hard minerals.
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