Hardness plays a major role in identifying a mineral. It can make the
identification process much simpler by considerably narrowing a search.
Hardness is defined by how well a substance will resist
scratching by another substance. For example, if mineral A scratches mineral B, and
mineral B does not scratch mineral A, then mineral A is harder than mineral B. If mineral
A and B both scratch each other, then their hardness is equal. A scale to
measure hardness was devised by Austrian mineralogist Frederick (Friedrich) Mohs in 1822,
and is the standard scale for measuring hardness. The scale consists of numbers
one through ten; 1 being the softest and 10 being the hardest. Each number represents a
different mineral - each harder than the previous. The 10 minerals are:
- Talc
- Gypsum
- Calcite
- Fluorite
- Apatite
- Feldspar
- Quartz
- Topaz
- Corundum
- Diamond
All conceivable minerals fit in this scale, since Talc is
the softest known mineral and Diamond the hardest. To demonstrate how to use the scale,
understand the following example: Suppose a mineral scratches Fluorite, but not
Apatite,
then it has a hardness between 4 and 5.
Several common household items have a fixed hardness, and
can be used to test for hardness:
| Fingernail |
2½ |
| Penny |
3 |
| Knife blade |
5½ |
| Glass |
5½ |
| Steel file |
6½ |
| Streak plate (floor tile) |
6½ |
Hardness is almost always rounded off to the nearest half
number.
There are various hardness testing kits. One type consists
of 10 metal rods, each one containing a fragment of one of the minerals in the Moh's
scale. Another type consists of large, low cost specimens of the Moh's minerals, labeled and
stored in a wooden compartment box. The Diamond is either absent or a chip attached to a
small metal rod. (The Diamond is really unnecessary, since no minerals are between hardness
9 and 10.)
A mineral is struck with a metal rod or "testing
mineral" to test its hardness.
It is tested in the manner of the following example:
| Action |
Conclusion |
| Mineral struck with rod or
mineral number 4 (Fluorite) from the testing kit. Mineral gets scratched. |
Mineral must be
less than or equal to 4. |
| Mineral struck with rod or
mineral number 2 (Gypsum). Mineral does not get scratched. |
Mineral must be
between 2 and 4. |
| Mineral struck with rod or
mineral number 3 (Calcite). Mineral gets scratched. |
Mineral must be
between 2 and 3. |
Two minerals with equal hardness will scratch each other. This gives an advantage
to the hardness testing kit that includes real minerals over rods. One can
scratch the mineral from the kit instead of scratching a nice specimen. In
addition, one can also get more exact results by seeing if both minerals scratch
each other.
Minerals can be damaged and lose value if not scratched properly. If a mineral
testing kit is composed of minerals (as opposed to rods), it is preferable for
the testing kit mineral to be scratched over the specimen. If this cannot be
done, than the specimen has to be scratched. This should be done in an area
where a scratch will be less noticeable, since it will make a permanent mark.
Hardness can be easily detected without a "kit".
All one needs to know is the hardness of certain items (including the ones mentioned
above) and minerals in his collection. These can be used instead of purchasing a kit.
How to test using hardness
Hardness testing is done by "scratching" one mineral with the other. To get the most accurate results, a sharp edge should be
scratched against a smooth surface, on a small an area as possible. The scratch
should not be conducted on a surface that is coated, chipped, or weathered, for
it will give inaccurate results.
When a mineral is scratched, a permanent indentation is
created. Powder of the softer mineral will come off, and it will cover the scratch area.
This powder needs to be brushed away to see if the mineral really got scratched, or if the
powder of the softer mineral that was swiped across the specimen being tested created a
scratch-like marking. When minerals of similar hardness are scratched together, it is
difficult to tell which mineral (if not both of them) is really getting scratched because
of this.
Most minerals are anisotropic to a minor extent, meaning their
hardness varies in different directions. Kyanite is famous for this habit. When scratched
in one direction, it exhibits a hardness of 4 to 5. When struck from the perpendicular
direction, it exhibits a hardness of 6 to 7. Kyanite is the only mineral exhibiting such
strong anisotropism. In virtually all minerals, the anisotropism is so weak that it cannot
be determined.
Micromounts and small embedded crystals are very hard to
determine in terms of hardness. One may not be able to test for hardness because of the
small size.
