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HARDNESS
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41
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The
stone under examination may perhaps first be somewhat roughly
classified by its colour, cleavage, and general shape. One of these
standard stones is then gently rubbed across its surface and then
others of increasingly higher degrees, till no scratch is evident under
a magnifying glass. Thus if quartz ceases to scratch it, but a topaz
will do so, the degree of hardness must lie between 7 and 8. Then we
reverse the process : the stone is passed over the standard, and if
both quartz and topaz are scratched, then the stone is at least equal
in hardness to the topaz, and its classification becomes an easy matter.
Instead
of stones, some experts use variously-tempered needles of different
qualities and compositions of iron and steel. For instance, a
finely-tempered ordinary steel needle will cut up to No. 6 stones ; one
made of tool steel, up to 7 ; one of manganese steel, to 7-1/2 : one
made of high-speed tool steel, to 8 and 8-1/2. and so on, according to
temper ; so that from the scratch which can be made with the
finger-nail on mica, to the hardness of the diamond, which diamond
alone will scratch readily, the stones may be picked out, classified
and tested, with unerring accuracy.
It
will thus be seen how impossible it is, even in this one of many tests,
for an expert to be deceived in the purchase of precious stones, except
through gross care-
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