With
a little practice one may become so good a judge of the hardness of a
mineral, by its behavior towards an ordinary pocket-knife, that the
minerals of the scale below 7 may be dispensed with. Thus minerals of
the first two degrees of hardness may be scratched with the finger
nail; No. 3 can be deeply scratched with a knife; No. 4 less deeply and
easily; No. 5 still less so; while No. 6 is about the hardness of the
knife. No. 6 also scratches ordinary window glass. Upon No. 7 a knife
blade makes no impression, the steel rubbing off on the mineral. Steel
of the hardness of a file, however, scratches quartz slightly. These
tests are especially useful for distinguishing glass imitations from
gems of the hardness of quartz and higher. Instead of a file it is well
to use a point of hardened steel to avoid danger of injuring delicate
gems. Rubbing the gem, especially if cut, with an aluminum pencil? is a
still better means of testing hardness in the higher numbers of the
scale, as it involves no danger of injury to the stone. Upon soft
stones such a pencil leaves a conspicuous mark, but upon hard ones none
whatever. Minerals above 7 in hardness are harder than a file.
Corundum scratches all minerals except diamond, and diamond is the
hardest substance known.
Some
minerals, if crystallized, are somewhat harder in one direction than
another, the mineral cyanite being a notable illustration of this.
Ordinarily, however, the hardness of a mineral is about the same in all
directions.
Table showing hardness of gem minerals:
