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Color
Hardness is not a measure of fragility, but rather of resistance to scratching. The assessment of hardness varies with the measurement technique used. All minerals can be ranked in order of relative scratch ability. One can scratch various minerals against each other, and ar¬range them in sequence of increasing hardness: On a simple scale of 1 to 10, with 10 the hardest, any mineral would scratch any other mineral with a lower hardness number.
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Such
an experiment was first done by an Austrian mineralogist,
Friedrich Mohs, who picked ten common miner¬als
to represent the division points of the scale. The Mohs
scale has survived the decades since its inception and
is widely used today: it should be noted that while
diamond is the hardest known sub¬stance, and will
therefore scratch any other substance, it is also some¬what
brittle and can be shattered or split. |
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This
is perhaps the best example of the difference between hardness
and tenacity. In some directions in diamond, the atomic bonds
are extremely strong, creating great hardness. In other directions,
the bonds are relatively weak, and so diamond has perfect
cleavage.
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DIAMONDS,
GEMSTONES & CRYSTALS
FACTS & FIGURES |
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use as a gemstone, a mineral should ideally have
both great hardness and tenacity. A brittle substance
may chip badly with wear, and ultimately slip
out of a setting. Even the act of setting a fragile
gem may cause it to break. Relatively soft gems
may show signs of wear after a short time, because
they are easily scratched by debris in the air
that accumulates and rubs against them.
Eventually
the surfaces of such a stone are so worn that
brilliance and transparency are lost, and repolish¬ing
is necessary.Hardness tests are never performed
on faceted gems because of the risk of damage.
Hardness is used more frequently in testing rough
and opaque gems, where a scratch on the back will
not be visible.
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Refraction
travels through space at a constant speed of about
186,000 miles per second. It is not slowed down
very much by air, but in solids and liquids it
is both slowed and forced to change its path.
The degree to which light is slowed down in a
material, relative to air, is called the aide,,
of refraction of the material. This index is also
a measure of the bending of light in a solid or
liquid medium. |
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Gems
cut from highly refractive materials tend to be brilliant
and bright, and this factor some¬times affects the style
of cutting of particular gems.
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| diamond 1 | 2 | 3 | 4 | 5 |
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