Hardness
Hardness is probably the most important or practical way to identify a
mineral. Hardness is the resistance of a mineral to scratching by another
substance (or mineral) of known hardness. A harder mineral can scratch
a softer one. Testing the hardness is often called “the scratch test”. A
simple numerical scale was devised by a German mineralogist,
Frederich Mohs, who arranged ten common minerals in order of
increasing hardness from the softest known (talc) to the hardest
(diamond) and gave each a “hardness number”. This scale is only
relative, not linear; the difference in hardness between any two minerals
on the scale is not quite the same. Gypsum (H=2) is only slightly harder
than Talc (H=1) but Diamond (H=10) is much harder than Corundum
(H=9).
Geologists often use objects close at hand to test for hardness. With a
fingernail (H=2 to 2.5), a penny (H=3 to 3.5) and a steel nail or needle
(H=5) the hardness of most minerals can be determined. Carrying a
small piece of quartz (H=7) extends the range. A hardness greater than 7
becomes very diagnostic as there are very few minerals in that range.
Tips:
The scratch test must be done with care. A common mistake is not to
recognize which mineral is doing the scratching. Rubbing a softer
(known hardness) mineral across a harder (unknown mineral) surface
may produce what looks like a scratch but really is a streak of fine
powder. If the “streak” rubs off then it is just that, a streak, and not a
scratch. Being able to rub away the scratch means the unknown mineral
is harder than the test mineral.
 Always rub away any loose powder before examining the scratch.
 When in doubt, use a magnifying glass to check that a scratch was really made.
 If your first test with a mineral of known hardness did not scratch the unknown
mineral then try a harder test mineral and move up the Mohs scale until you do
produce a scratch.
Mohs Hardness Scale
1 Talc
Talc has a Mohs hardness of 1, and usually has a soapy or greasy feel because it is
so soft. It scratches very easily with a fingernail. Ground up talc is often used as a
dusting powder or talcum.
2 Gypsum
Gypsum is a calcium sulphate mineral, often used for making gyprock or wall
board for houses and buildings. It occurs in many colours from white to pale pink
to red, yellow or brown and it can have a lustre from dull to silky to almost
vitreous. Its most characteristic property is a Mohs hardness of 2, it can be
scratched with a fingernail but is noticeably harder than is talc.
3 Calcite
Calcite or calcium carbonate is one of the most common minerals on the crust of
the earth and it appears in a multitude of colours and from very fine-grained
limestone to pegmatitic single crystals. It has a Mohs hardness of 3 and when
tested with acid such as vinegar or very dilute hydrochloric acid will fizz strongly
and produce a profusion of bubbles.
4 Fluorite
Fluorite is a calcium fluoride mineral with a Mohs hardness of 4. The colours of
greenish varieties of fluorite are similar in colour to apatite but apatite has a
hardness of 5, close to that of a steel needle whereas fluorite is easily scratched by
a steel needle or nail. Dark purple fluorite is readily distinguished from dark purple
amethyst, which is quartz with a Mohs hardness of 7.
5 Apatite
Apatite This specimen is an elongated or prismatic single crystal with six sides, it
is hexagonal. It has a Mohs hardness of 5.
6 Orthoclase
Orthoclase feldspar has a hardness of 6. This reddish brown specimen appears to
be banded with bits of white and is actually two slightly different feldspars which,
together, are called Perthite after the town of Perth, Ontario where it was first
found.
7 Quartz
Quartz has a Mohs hardness of 7 and is found in rocks of all ages and in many ore
deposits. Quartz is the most common mineral of all. It is a major constituent of
many igneous, metamorphic and sedimentary rocks and is commonly found as
loose grains on sand beaches or river bottoms. It is harder than most minerals and
this is often the characteristic that allows one to identify it. The hardness does not
change whether one is looking at a single large crystal, massive quartz from an ore
vein or at agate/chert or tiger-eye.
8 Topaz
Topaz has a Mohs hardness of 8 and if often difficult to distinguish from quartz.
The hardness is the “give-away” as it will scratch quartz, one of the very few
minerals that will do so.
9 Corundum
Corundum is harder than Topaz, and has been given a Mohs hardness of 9. Is also
known as Emery and has been used for special sandpaper (emery paper) when
great hardness is required. Coloured varieties are known as ruby and sapphire. The
only mineral capable of scratching corundum is diamond itself.
10 Diamond
Diamond is hardest mineral known with a 10 on the Mohs scale. It is pure carbon
just as is graphite but the bonds in diamond are all very strong and equal to each
other so the mineral is very strong. The specimen in this image and on display in
our museum is a glass replica of a cut diamond that would be worth many
thousands of dollars if it were real.