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| Mohs |
A practical method for evaluating the hardness of a gem
was proposed by Friedrich Mohs. He selected a sample of
ten noted and easily found gems, ordered them based on
their resistance to abrasion and compared them to other
materials. In this manner, a scale of hardness was created
and named the Scale of Mohs in his honour. The ten minerals
of the scale of Mohs, which mineralogists and gemmologists
universally accept as standards of hardness are:
- 10 Diamond
- 9 Corundum
- 8 Topaz
- 7 Quartz
- 6 Orthoclase
- 5 Apatite
- 4 Fluorite
- 3 Calcite
- 2 Gypsum
- 1 Talc
The numbers indicate the levels of hardness of the scale
of Mohs and are conventional values that denote a diminishing
order of hardness from 10 to 1. |
| The cardinal virtues |
| The value of a stone is classified based on an analysis
of the three cardinal virtues of gems: beauty, durability
and rarity. The first virtue is beauty: the beauty of
a stone is determined either by its transparency and
the intensity of its colour (as with the ruby, sapphire
and emerald), its colour alone (as with turquoise), or
by the dispersion of its white light into the colours
of the spectrum, which is responsible for its “fire” (most
evident in the diamond). The beauty of a gem, however,
is largely hidden until it is brought out by a lapidary
(gem cutter). The second virtue, durability, is connected
to a gem’s
hardness and determines its ability to withstand abrasive
and chemical attacks, and therefore its resistance to
wear and tear that could ruin and destroy its brilliance.
Three aspects of durability are directly correlated to
the intrinsic physical properties of a given gem variety:
hardness, tenacity and stability. Hardness is defined
as the level of a material’s
resistance to cutting or scratching. It is classified
by the Mohs scale (see diagram). Tenacity is a gem’s
ability to resist breaking, chipping and abrasions. Lastly,
stability refers to the generic properties of a gem that
include both its reaction to heat and abrupt temperature
changes, as well as its reaction to radiation (including
light) and chemical agents. The third virtue, rarity,
is often the most important! The law of supply and demand,
which is often influenced by the whims of fashion, largely
determines the rarity of a gem. Have you ever wondered
why gems are rare? What is it that makes gems difficult
to find and therefore more precious than any other type
of mineral? The answer to these questions can be explained
by our unique natural environment and the exceptional
combination of conditions that must exist in order for
a gem to form. |
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