A good property in mineral identification is one that does not vary from
specimen to specimen. In terms of reliability, hardness is one of the better
physical properties for minerals. Specimens of the same mineral may vary
slightly from one to another, but generally they are quite consistent.
Inconsistencies occur when the specimen is impure, poorly crystallized, or
actually an aggregate and not an individual crystal.
Hardness is one measure of the strength of the structure of the mineral relative
to the strength of its chemical bonds. It is not the same as brittleness, which
is another measure of strength, that is purely related to the structure of the
mineral. Minerals with small atoms, packed tightly together with strong covalent
bonds throughout tend to be the hardest minerals. The softest minerals have
metallic bonds or even weaker van der Waals bonds as important components of
their structure. Hardness is generally consistent because the chemistry of
minerals is generally consistent.
Hardness can be tested through scratching. A scratch on a mineral is actually a
groove produced by microfractures on the surface of the mineral. It requires
either the breaking of bonds or the displacement of atoms (as in the metallic
bonded minerals). A mineral can only be scratched by a harder substance. A hard
mineral can scratch a softer mineral, but a soft mineral can not scratch a
harder mineral (no matter how hard you try). Therefore, a relative scale can be
established to account for the differences in hardness simply by seeing which
mineral scratches another. That is exactly what French mineralogist Friedrich
Mohs proposed almost one hundred and seventy years ago. The Mohs Hardness
Scale starting with
talc at 1 and ending with
diamond at 10, is
universally used around the world as a way of distinguishing minerals. Simply
put; the higher the number, the harder the mineral.
Below is the Mohs Hardness Scale:
In order to use this scale, it is necessary to have on hand some of the minerals
in the scale. If you wish to test an unknown mineral for hardness you might want
to start with an ordinary specimen of apatite to see if the unknown mineral can
scratch it. If the unknown mineral scratches the apatite, then you can conclude
that it has a hardness of 5 or more. If the apatite can scratch the unknown
mineral, then the unknown mineral has a hardness of 5 or less. If they can
scratch each other, then the unknown mineral has a hardness of 5. You will need
to perform other tests to narrow down the hardness. If it is softer than
apatite, try calcite, etc., etc until you have narrowed down the approximate
hardness. Remember, this is a relative scale and a mineral that can scratch a
mineral that has a hardness of 4.5 may be given a hardness of 5, but it still
might be softer than apatite.
- Corundum (ruby
One word of caution for inexperienced collectors: do not SCRATCH NICE CRYSTAL
A fractured, cleaved or inconspicuous part of the mineral should still give a
good hardness test and not damage a potentially wonderful specimen.
What if you do not have the minerals listed in the Mohs Hardness Scale? Well, a
collector might keep a few items of known hardness in a "hardness kit";
just in case they are needed.
Below is a revised Mohs Hardness Scale with some
everyday items listed:
Again, the Mohs Hardness Scale is only relative. Meaning that fluorite at 4 is
not twice as hard as gypsum at 2; nor is the difference between calcite and
fluorite similar to the difference between corundum and diamond. An absolute
hardness scale looks a little different than the relative scale. Using sensitive
equipment, a comparison of the absolute hardness of minerals can be measured. It
turns out that most minerals are close in hardness. But as hardness increases,
the difference in hardness greatly increases as seen in the scale below.
- --window glass or typical knife blade at under 5.5
--streak plate or good steel file at over
Below is an absolute hardness scale:
It is easy to see why diamond gets so much respect as the hardest natural
substance known to man. The next hardest mineral, corundum, is four times softer!
There are many substances that are currently being created and studied to beat
diamond in hardness. But diamonds' all carbon, extremely dense, structurally
sound and tightly bonded structure is hard to beat. At present only diamonds
created with isotopes of carbon have exceeded the mark of 10 on the hardness
Hardness is particularly important for
Very few soft minerals are cut as gems and when they are, they generally are cut
only for collectors and not for wearable jewelry. Apatite is one of the softest
of gemstones. Mostly gemstones have a hardness of 7 or more. Hardness also plays
a major apart in the minerals that are used for grinding, polishing and other
abrasive purposes. Soft minerals can be used as high temperature lubricants,
pencil lead, talcum powder, paper gloss, etc.
Here are a few hints on how to use
hardness to identify minerals:
- Keep in mind that most minerals have small differences in hardness according
to the direction of the scratch and the orientation of the scratch and that some
minerals such as
diamond, have a
marked difference in hardness with respect to these factors.
- A massive specimen will probably be softer than a single crystal and ideally
hardness should only be used on individual crystals.
- Some minerals will have a range of hardness due mostly to impurities or
substitution of certain ions.
- Don't be fooled by a dust trail on a mineral after being
"scratched" by a softer mineral. It may look as if the softer mineral
scratched the harder mineral but in actuality the softer mineral just left a
dust trail across the resistant surface of the harder mineral (always blow air
over the scratch or rub across the scratch to be sure it's a scratch).
- Another clue to relative hardness is ease of scratching (both diamond
and quartz scratch glass, but diamond scratches glass ". . . like a knife
- To remember the Mohs scale try remembering this phrase:
Quartz, (that) Tourists