Some minerals are soft and can be scratched easily. Others, which are
harder, are resistant to scratching. To measure a mineral’s hardness, we
try to find out which substances will scratch it and which substances
will not scratch it. To do this in a general way, several ordinary
objects—such as a fingernail, a copper penny, a pocket knife, a piece of
window glass, and a steel file—can be used. For a more exact way of
testing hardness, we can use ten minerals that make up what is known as
_Mohs scale_. Each mineral in this scale has a different hardness, and
each one has been given a number that represents its hardness. For
example, talc, the softest mineral in this scale, is given a hardness of
_1_. Gypsum, the next softest mineral in the scale, has a hardness of
_2_. Diamond, the hardest mineral known, is given the top hardness of
_10_ in this scale. These ten minerals are listed below. Alongside them
are five common objects with their hardnesses.
1—Talc
2—Gypsum Fingernail—slightly over 2
3—Calcite Copper penny—about 3
4—Fluorite
5—Apatite Pocket knife—slightly over 5
6—Orthoclase Window glass—5½
7—Quartz Steel file—about 6½
8—Topaz
9—Corundum
10—Diamond
Suppose, for example, that a mineral can be scratched by fluorite, which
has a hardness of _4_ on Mohs scale, but cannot be scratched by calcite,
which has a hardness of _3_. We then know that this mineral is softer
than fluorite, but harder than calcite; therefore, it has a hardness of
about _3½_. In the same way, if a mineral can be scratched by a pocket
knife, which is slightly more than _5_ in hardness, but not by a copper
penny, which has a hardness of about _3_, we know then that its hardness
is between _3_ and _5_.