ade use of in identifying it.

No scale of _absolute_ hardness has ever come into general use, but the
mineralogist Mohs many years ago proposed the following _relative_
scale, which has been used very largely:

MOHS'S SCALE OF HARDNESS. Diamond, the hardest of all gems, was rated as
10 by Mohs. This rating was purely arbitrary. Mohs might have called it
100 or 1 with equal reason. It was merely in order to represent the
different degrees of hardness by numbers, that he picked out the number
10 to assign to diamonds. Sapphire (and ruby) Mohs called 9, as being
next to diamond in hardness. True topaz (precious topaz) he called 8.
Quartz (amethyst and quartz "topaz") was given the number 7. Feldspar
(moonstone) was rated 6, the mineral apatite 5, fluorspar 4, calcite 3,
gypsum 2, and talc 1.

It may be said here that any mineral in this series, that is of higher
number than any other, will scratch the other. Thus diamond (10) will
scratch all the others, sapphire (9) will scratch any but diamond, topaz
(8) will scratch any but diamond and sapphire, and so on.

It must not be thought that there is any regularity in the degrees of
hardness as expressed by these numbers. The intervals in hardness are by
no means equal to the differences in number. Thus the interval between
diamond and sapphire, although given but one number of difference, is
probably greater than that between sapphire (9) and talc (1). The
numbers thus merely give us an order of hardness. Many gem minerals are,
of course, missing from this list, and most of the minerals from 5 down
to 1 are not gem minerals at all. Few gem materials are of less hardness
than 7, for any mineral less hard than quartz (7) will inevitably be
worn and dulled in time by the ordinary road dust, which contains much
powdered quartz.

In testing a gem for hardness the problem consists in finding out which
of the above minerals is most nearly equal in hardness to the unknown
stone. Any gem that was approximately equal in hardness to a true topaz
(8) would also be said to be of hardness 8. Thus spinel is of about the
same hardness as topaz and hence is usually rated as 8 in hardness.
Similarly opal, moonstone, and turquoise are of about the same hardness
as feldspar and are all rated 6.

Frequently stones will be found that in hardness are between some two of
Mohs's minerals. In that case we add one half to the number of the
softer mineral; thus, peridot, benitoite, and jade (neph