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HARDNESS
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Tests
of hardness afford one of the most useful and convenient means of
distinguishing gems. Such tests can be easily made and are very
reliable, the hardness of species being remarkably constant. Hardness
should not, however, be confounded with toughness, i. e., the
difficulty with which a mineral can be broken, since many brittle
minerals have considerable hardness. Hardness is rather the power of
resistance to scratching which a mineral possesses.
It
is evident that a high degree of hardness must be an important property
of precious stones, as their polish would soon disappear if they were
easily scratched.
The
common method of stating the hardness of a mineral is by referring it
to its place in the scale devised by the German mineralogist Mohs. The
divisions of this scale are constituted by ten rather common minerals,
arranged according to their hardness. The scale is as follows:
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To assist in remembering the minerals of this scale in their order, the following mnemonic has been devised:
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The
position of a mineral in this scale is determined by the minerals which
it scratches. Thus if a mineral scratches feldspar, but is scratched by
quartz, its hardness would be stated as 6.5. In order to test
hardness, pieces of the minerals of the scale should be at hand.
Fragments of the mineral to be tested may be grasped in the fingers and
rubbed upon a polished surface of the minerals of the scale, or the
test can often be more accurately made by rubbing upon the mineral of
the scale a coarse powder of the mineral to be tested, by means of a
soft pine stick.
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