Hardness
is an important character of gem-stones, because the durability of the
gem-stones depend on this character. Hardness is defined as the
resistance which a mineral offers to abrasion or scratching. When a
crystal or quartz is used to scratch a crystal of calcite, a deep
scratch and white powder are left on the calcite. When it is reversed
no scratch is seen on the quartz. Minerals are tested in this manner
and arranged according to their hardness. A scale of hardness in
ascending order has been devised to fix the hardness of minerals, known
as Moh's scale of hardness, which is given below.
(1) Talc (6) Felspar (Orthoclase)
(2) Gypsum (7) Quartz
(3) Calcite (8) Topaz
(4) Fluorite (9) Corundum
(5) Apatite (10) Diamond
The values assigned are only relative hardness. Other supplementary tests for hardness are as follows:—
Finger nail up to 2-1/2, copper coin up to 3, knife blade up to 5 -1/2, window glass 5-1/2, steel file 6-7.
Gem-stones
have their hardness at the top of the above list; these minerals are
sold as a scale of hardness. To find the hardness of a stone, we must
find the highest mineral in the scale that it will scratch and the
lowest by which it can be scratched. Then the hardness of the mineral
or gem-stone could be fixed. Hardness tests require some practice and
are not simple. The power of abrasion will also depend on the sharpness
of the scratching point and the smoothness of the surface scored. The
hardness of rough or cut gems is best done by hardness pencils or
points. They are holders with conical shaped fragments of the test
minerals mounted on ends. When applying this test the scratch should be
as short as possible and should not be on a prominent face. The scratch
should be very small and using a lens with an
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