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FIRE IN THE EARTH
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7. Quartz 9. Corundum
8. Topaz 10. Diamond
The
numbers in the scale indicate merely the order of hardness, not
quantitative value. A diamond (10), for instance, doesn't necessarily
have twice the hardness of apatite (5), nor does the topaz (8) have 80
per cent of the hardness of the diamond, not by a long stretch of the
scale. Indeed, the hardness interval between diamond and corundum is
far greater than that between corundum and talc, the softest of mineral
substances.
To
test the hardness of a stone it was necessary to discover the highest
mineral in the scale that it would scratch, and the lowest mineral by
which it itself could be scratched. Hundreds of years ago the scratch
test wasn't used; the ignorant Hindus decided hardness with the blow of
a hammer, thus shattering a lot of perfectly valuable stones just to
formulate a theory or to sustain one. But in scratching it was found
that only a diamond could scratch a diamond; a knife point could
scratch feldspar but not quartz; and the harder a stone is the more
readily it can cut glass. It takes only a fingernail to scratch talc
and gypsum. And, of course, a harder mineral scratches a softer one. So
if a mineral scratches all other minerals up to quartz, and it is
scratched by all minerals down to topaz, then the hardness lies between
numbers 7 and 8 on the scale. If it scratches quartz about as readily
as it is itself scratched by topaz, then the hardness can be placed at
7-1/2. For the layman, however, the safest and most economical thing is
to take a mineralogist's word for these things and not go around
collecting valuable stones and scratching the value out of them; there
are other less expensive and destructive hobbies in this world. But one
good comes out of all this, for hardness is a char-
(6)
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