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.
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.
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 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
When a gemstone
breaks along a surface that is not related to its internal atomic structure,
it is said to fracture.
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
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.
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.
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