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Physical Properties of the Gemstones ….
The mineral's composition and crystalline structure impart the various
physical properties that characterize each specimen. Knowledge of the
properties of gemstones is important for the gem cutter and setter, as well as
to the consumer who can use that information to care for the gem.
A perfect crystal is bounded by plane faces which meet at angles specific for
each kind of material (angle analysis can identify minerals). A crystal may be
cleaved in directions related to the external form or to a possible crystal form
for the mineral. Sometimes two distinct minerals can have the same chemical
composition with their differing properties being due to their different
crystal structure. Crystal structure affects mineral properties more than their
chemical nature. Examples here include diamond (carbon, cubic) and graphite
(carbon, hexagonal) and Calcite (trigonal) and aragonite (orthorhombic), both
forms of calcium carbonate.
Specific Gravity:
The specific gravity of a gemstone is the ratio of the weight of the material
to the weight of the same volume of water at a temperature of 4 degrees
Celsius. In general, minerals composed of heavy elements will have a higher
specific gravity than those composed of lighter elements, although bonding
and crystalline structure can also affect the specific gravity. Also, the more
closely packed the atoms, the stronger the bonding, and the higher the
specific gravity. Heavier gemstones are usually harder as well. The range is
from amber, which has a specific gravity of 1.08 and opal, with a specific
gravity of 2.05, all the way up to corundum (sapphires and rubies) with a
specific gravity of 3.99, spessartite garnet, specific gravity of 4.15,
marcasite, specific gravity of 4.9, and cuprite (s.g., 6.0) and casseterite (s.g.,
6.9). Diamond is in the heavy mid-range, with a specific gravity of 3.52.
Ways of measuring Specific Gravity
There are several ways to directly measure the specific gravity. To arrive at a
relative measure of specific gravity, heavy liquids are used. Gems are placed
in liquids of a known specific gravity. If the gem floats, its specific gravity is
less than that of the liquid; if it sinks, the gem is heavier than the liquid; and
if the gem remains suspended, it is very close to the liquid's known specific
gravity. Another useful specific gravity liquid is saturated salt solution (SG
= 1.08) which is used to separate amber from most plastic imitations. Amber
will float and the plastic imitations will sink.
The hardness of the mineral refers to its resistance to scratching and abrasion
and also to the cutting resistance. The more resistant the surface is to
scratching, the harder the mineral, and the stronger the bonding forces are
holding the atoms together. 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. This scale was devised by an Austrian, Friedrich Mohs, and runs
from talc, the softest (H=1), and diamond, the hardest (H=10). Simply stated
a harder mineral will scratch a softer one, and minerals of the same hardness
will scratch each other. Gems with a hardness of 2 or less are considered soft;
those with hardness 3 to 5 are called medium; gems with hardness of 6 and
over are hard (Schumann, 1997, p. 19).
Cleavage and Fracture:
Cleavage and fracture refer to the characteristic manner in which gems will
break when an external force or stress is applied. Some minerals have a
special way of breaking parallel along planes of atomic weakness, creating
smooth flat surfaces. This break is called cleavage. Crystalline minerals have
cleavage and fracture, whereas amorphous or massive stones only fracture.
In rough material, a cleavage break may already be obvious or it can be
determined by giving the specimen a tap with a hammer. Rough diamond is
often cleaved and then cut into shapes. Cleavage is not possible to observe in
fashioned gems unless an internal imperfection can be observed or there is an
accidental blow struck along a cleavage direction and the gem breaks. Thus,
diamond has very well developed cleavage and although it is the hardest
known substance, the ready cleavage makes it susceptible to damage.
Tenacity or Toughness:
Tenacity or toughness is the ability of a stone to withstand pressure or
impact. It is the resistance to crushing, breaking, or tearing. Minerals which
crumble into small pieces or a powder are said to be brittle. Tenacity terms
include flexible, elastic, malleable, sectile, and ductile. 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. The jade gemstones (jadeite,
nephrite) are the toughest of all gems, making them also difficult to cut. Talc
and gypsum are examples of minerals which are flexible. Ductile or malleable
minerals are those (gold, silver, etc.) which may be flattened out into thin
sheets under pressure. The brittleness factor of a gemstone is an important
consideration in gem cutting and polishing. Many gem crystals shatter or
chip easily, and this must be taken into consideration when cutting.
Diamond is the hardest known substance but because of well developed
cleavage and a brittle tenacity, it can easily shatter when hit.
The degree of tenacity
exceptional - like nephrite and jadeite jade
excellent - like corundum
good - like quartz
fair - like tourmaline
poor - like topaz
A fair or poor tenacity does not mean the gem is less valuable, but does have
implications for care and cleaning as well as setting the stone in a secure,
protective mounting.
Magnetism and Electricity:
Those stones which are attracted by a magnet are considered magnetic, such
as magnetite and hematite, which contain iron. Hematite, an imitation of
hematite, is magnetic, whereas most natural hematite is very weakly
magnetic. Synthetic diamond can contain iron-nickel flux inclusions and can
show magnetism (when floating in a heavy liquid such as Cleric’s solution),
whereas natural diamond exhibits no magnetism.
Piezoelectricity, or pressure electricity, is found in minerals that have polar
axes or lack a center of crystalline symmetry. The crystal axes have different
properties at the opposite ends of the polar axis, and when pressure is exerted
at these ends, electricity can flow creating opposite positive and negative
ends. Quartz and tourmaline are piezoelectric.
Frictional electricity:
Frictional electricity, or an electrostatic charge created by rubbing, is
common in many gems. The ability of the gem to attract light objects is
dependent upon the charge and was probably first recognized in amber more
than 2500 years ago. The Greek name for amber is "elektron," origin of our
word electricity.
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. Heat is
conducted differently in various minerals according to their crystal system. 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.