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44 PRECIOUS STONES
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sound
and the less the powder. An example of this difference is evident in
the cutting of ordinary glass with a " set " or " glazier's " diamond,
and with a nail. If the diamond is held properly, there will he heard a
curious sound like a keen, drawn-out " kiss," the diamond being
considerably harder than the material it cut. An altogether different
sound is that produced by the scratching of [glass with a nail. In this
case, the relative difference in hardness between the two is small, so
that the glass can only he scratched and not " cut " by the nail ; it
is too hard for that, so the noise is much greater and becomes a
screech. Experience, therefore, makes it possible to tell to a trifle,
at the first contact, of what the stone is composed, and in which
class it should be placed, by the mere " feel" of the scratch, the
depth of it, the amount and kind of powder it leaves, and above all, by
the sound made, which, even in the tiniest scratch, is quite
characteristic.
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