Physical Properties of Gemstones
The physical
properties of gemstones, their hardness, their specific
gravity or density and they way they break, depend on
chemical bonding and the atomic structure within the
stone.
Specific Gravity of Density
The specific gravity of a gem is
its weight when compared with the same volume of water
at a temperature of 4 degrees Celsius. The denser the
minerals in the gemstone are, the heavier the weight or
specific gravity will be. Heavier gemstones are usually
harder as well.
Hardness
Gemstones are often tested by
using the Mohs’ hardness scale to determine just how
hard they are. The harder minerals are more durable in
that they do not scratch easily and will hold up better
in jewelry. Talc is the softest mineral with a hardness
of 1 and can be easily scratched with a fingernail. The
gemstones with a rating of 7 or over are relatively
hard. Quartz gemstones (citrine, amethyst, etc.) range
in the 7's, topaz rates 8, and corundum (sapphires and
rubies) are a 9 on the Mohs' hardness scale. Diamond
registers a 10 and is the hardest known naturally
occurring material on earth, more than ten times the
hardness of corundum at 9. There is more of a spread
between the gems and minerals found between 2 and 3 and
between 5 and 6, however corundum is only about 10 per
cent harder than topaz.
Cleavage and Fracture
Cleavage is the splitting of gems
and minerals along one of the planes related to the
stone's structure. Crystalline minerals have cleavage
and fracture, whereas amorphous or massive stones only
fracture.
Fracture is the way a stone breaks. Consider fracture to
be similar to a piece of wood breaking in a direction
other than the direction of it's grain. Conchoidal
fracture, which is most common in gemstones, shows a
series of arcs that spread outward.
When a gemstone breaks along a surface that is not
related to its internal atomic structure, it is said to
fracture.
Tenacity or Toughness
Tenacity or toughness is the
ability of a stone to withstand pressure or impact.
Minerals which crumble into small pieces or a powder are
said to be brittle. If a gem bends but returns to its
original position, it is said to be elastic (mica,
nephrite, jadeite); these minerals are tough and
difficult to break.
Magnetism & Electricity
Those stones which are attracted
by a magnet are considered magnetic, such as magnetite
and hematite, which contain iron. Most minerals and gems
are poor conductors of electricity. Good natural
conductors include native metals and minerals with a
metallic luster (pyrite). Natural blue diamond is a
semi-conductor.
Thermal Conductivity
Some stones are good conductors
of heat, such as quartz, which draws heat away from the
body when held and thus feels cold to the touch. A poor
thermal conductor, such as amber, feels warm to the
touch because it does not conduct heat away from the
body. The surface of a genuine gemstone will de-mist
more rapidly than that of glass or an artificial stone.
Crystal Systems
Although traditionally crystal
systems are not part of the physical properties table of
gemstones, but most mineral gemstones are crystalline,
with their atoms arranged in regular and symmetrical
patterns. Crystal systems are classified into seven
different systems, according to the "minimum
symmetry" of their faces.
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