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42 PRECIOUS STONES.
VI. Hardness.
All
the true gems are essentially hard. A mineral does not fulfil the
commonly accepted idea of a gem unless it is hard ; but the degree of
hardness varies considerably. An arbitrary scale devised by Mohs is
used to express the relative hardness of different minerals. Ten
different minerals of dissimilar hardness are chosen ; these are (No. 1
being the softest) —
1. Talc.
2. Gypsum.
3. Calcite.
4. Fluor Spar.
5. Apatite.
6. Orthoclase Felspar.
7. Quartz.
8. Topaz.
9. Sapphire. 10. Diamond.
If
we were dealing with an uncut gem on which a scratch would not be of
great importance we might first apply a sharp corner of it to the test
stones, beginning with the softest until we come to one it will not
scratch ; if on reversing the positions the test stone does not scratch
the stone under examination, we know that the two are of equal hardness
; should it scratch the examined stone we know the latter has a
hardness between the test stone that will scratch it and the test stone
next in the series below. By approximation we may fix the hardness at,
say, 7"25 or 7"5 when the examined stone just scratches Quartz and is
easily scratched by Topaz. In the case of cut gems we usually have to be
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