Colored Stones : Physical Properties
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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
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.
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 effect
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.
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.
There are drawbacks to these heavy liquids though. All
of the heavy liquids used to determine specific gravity are poisonous
and breathing the vapors is not advised. Also gems suspectible to
chemical attack, such as amber or hematite, could be damaged using
this suspension method.
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).
Only 10 or 12 of the major gemstones have the ideal hardness or a
hardness greater than 7. 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.This ideal hardness designation
stems from the fact that quartz (H=7) is the most abundant mineral on
Earth and present as tiny particles in the dust that settles on
jewelry, which can lead to scratching and abrasion. Therefore, dust
may dull the luster and polish of gems with hardness of 7 or less.
Diamond registers a 10 and is the hardest known naturally occurring
material on earth, more than ten times the hardness of corundum at 9.
Talc is the softest mineral with a hardness of 1 and can be easily
scratched with a fingernail. 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. The hardness is
relative, but it is, nevertheless, a useful identification tool.
Hardness is almost never used as a separation test with gemstones
since it is considered a destructive test and other nondestructive
tests exist to enable separation and identification.
Hardness testing is acceptable with some rough material, but rarely
done on fashioned gems. It is a test that is never used on transparent
stones. It is a destructive test, which separates atoms and actually
leaves a groove on the specimen.
For the gem cutter, a knowledge of hardness is important.
Because hardness is related to bonding, different hardness can occur
on the same gem in different directions, which means hardness can have
an effect on durability as well as beauty. Harder minerals will result
in sharper facet junctions and take a better surface polish.
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 suspectible to damage.
A knowledge of cleavage for the cutter is important as it can lead to
an easy first step to the fashioning process for diamonds. When
considering colored stones, cleavage is avoided as it is very
difficult to polish a gem parallel to a cleavage plane (Hurlbut and
Kammerling, 1991, p. 54). The heat produced when soldering the setting
can cause fissures along cleavage planes and may lead to the gem
actually breaking along these fissures (Schumann, 1997, p. 22).
Piercings or drilling should be done vertically to the cleavage
surfaces (Schumann, 1997, p. 22).
Fracture is the way a stone breaks. It is a break in a direction
other than along cleavage planes and results when the bonding forces
are similar in all directions. Consider fracture to be similar to a
piece of wood breaking in a direction other than the direction of it's
grain. A distinctive, common fracture is called conchoidal, which is a
shell-like break. This break is seen in glass, quartz, opal, peridot,
and amber, to name a few. Other possible fractures include uneven,
splintery, granular, or subconchoidal.
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
- like nephrite and jadeite jade
- like corundum
- like quartz
- like tourmaline
- 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. Hematine, 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 Clerici's solution), whereas natural diamond exhibits no
The ability of a mineral to conduct electricity is referred to as
electroconductivity. This property is mostly characteristic of
minerals with metallic bonding, such as gold, silver, and copper.
Minerals with partial metallic bonding are semiconductors of
electricity. Most gem minerals lack metallic bonding and thus are
nonconductors, with the exception of natural and synthetic blue
diamonds that do conduct electricity. Blue diamonds that are colored
by artificial irradiation are electrical insulators and can be
separated from naturally colored and synthetic blue diamond with
thermal inertia meters (electrical conductometers).
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. Thin slices of
quartz oscillate when subjected to alternating current, controlling
radio frequencies of electronic circuits for radios (since 1921) and
watches (Hurlbut and Kammerling, 1991, p. 64). Tourmaline has been
used in pressure gauges since 1945, when the blast pressure of the
first atomic bomb was measured.
Pyroelectricity, or heat electricity, occurs
in minerals with polar axes or lack the center of crystalline
symmetry. As a function of temperature, such as display lighting or
heat in a display window with sun, positive and negative charges can
build up in some gems. This means tourmaline can attract dust
particles more easily when heated.
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
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.
Thermal conductivity should also be considered when cutting
gemstones, as some stones will need a cooling-off period during the
cutting. This is also used in Thermal Conductivity instruments to
differentiate diamond which conducts heat very well from its simulants
and imitations. Some instruments use it to identify other gemstones
but they are expensive and of value only when used with care and some
gemmological knowledge. The use of standard stones is suggested and
drafts to be avoided as they can change the readings. At its simplest
this is the temperature test using tongue or lips for glass and
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