In the first chapter of SDI’s Gemology Guide we dealt with the
cardinal virtues of a gemstone, now moving further we will explore the nature
of these colorful and vibrant gems. In other words we will get to know about
the properties of gemstones. The basic properties of gems have an extremely
important bearing for not only who wish to study gemology but also for gem
cutters, buyers and for people who deal in the gemstone markets. Properties of
gemstones can be broadly classified into two: physical and optical. Physical properties are
determined most importantly by the crystal structure and chemical composition
of a gemstone whereas the optical properties are dependent on various other
factors along with the crystal structure.
This post will talk about the physical properties of a gemstone.
First and foremost, to have a crystal clear understanding of a gem’s
physical properties it is required to be acquainted with what is a crystal
structure of a gem. Each and every gem whether organic or inorganic is made up of atoms. These atoms are the basic building blocks for any
gem. Now, almost all the gems are inorganic minerals and these minerals always
grow in a specific order termed as the crystalline structure. The regular,
repeating arrangement of the atoms determine the crystal structure. The symmetry
of the arrangement of atoms defines the outer shape of the gem crystal. The crystal structure affects various
properties of a gem like hardness, cleavage, fracture, specific gravity etc.
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| AMETHYST CRYSTALS |
There are certain organic gems like opal and amber which are also
made up of atoms but these atoms lack a systematic arrangement. Hence these
gems are called amorphous gems.
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| OPAL-AMORPHOUS GEM |
Certain factors like heat, pressure, space for crystal growth,
chemicals available in the growing environment, cooling time etc, hugely affect
the growth of crystals. These are basic requirements for gem crystals to grow.
In any case if these requirements are not met it can lead to distortion in the
growth. The atoms can get arranged in random patterns, when this happens
aggregate gems are formed.
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| JADE-AGGREGATE GEM |
Classic example of this is Amethyst which belongs to Quartz group and
chalcedony which also belongs to the same group. Amethyst is a well formed
crystal of the quartz group whereas chalcedony is an aggregate gem which
results from disturbance which is caused
in the growth process of a crystal.
There are seven basic crystal systems in which the gem crystals grow.
Cubic – eg. Diamond, spinel, garnet etc.
Tetragonal – eg. Zircon
Hexagonal – eg. Berly and apatite
Trigonal – eg. Corundum, quartz, tourmaline
Orthorhombic – eg. Topaz, iolite, tanzanite, chrysoberyl, peridot
Monolinic – eg. Kunzite spodumene, moonstone orthoclase feldspar
Triclinc – eg. Amazonite, rhodonite,turquoise.
PHYSICAL
PROPERTIES
Hardness: one of the most important virtue of a gemstone is its
durability. But what makes a gem durable? Hardness of a gem is what makes it
durable. Hardness can be defined as the ability of a gem to resist scratches.
This property is greatly dependent on the crystal structure and how strong is
the bond between the atoms. Hardness is usually expressed through Mohs Scale
wherein various minerals are listed and given a number depending on its ability
to resist scratches. This scale is based on the concept that a hard mineral can
scratch another softer mineral. For example quartz can be easily scratched by
corundum.
MOHS SCALE
(top to bottom – hard to soft)
10. Diamond
9. Corundum
8. Topaz
7. Quartz
6. Orthoclase Feldspar
5. Apatite
4. Fluorite
3. Calcite
2. Gypsum
1. Talc
Toughness/Tenacity: This property is often confused with hardness but
these are two different concepts altogether. Toughness is the ability of
gemstone to withstand breaking and chipping. In other words how well can a gem
take physical stress. Aggregate stones like jade and nephrite are excellent
examples of tough stones. Topaz on the other hand is a very brittle stone.
Cleavage: it is a characteristic manner in which a gem breaks when an
external force or stress is applied to it. Usually the break comes parallel to
the area wherein the bond between the atoms is weak. This break creates smooth
flat surfaces along the plane of atomic weakness.
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| CLEAVAGE |
Parting: for one to understand what is parting in a gem, it is
required to understand the concept of twinning. Sometimes during the growth of
a crystal, the internal arrangement of atoms is disturbed. This disturbance is
caused by certain changes in the growing atmosphere of the crystal. This leads
to a change in the growth direction of the crystal forming twins. This process
is termed as twinning and the location where the direction of growth changes it
is called as twinning plane.
Twinning might take place not only when the crystals are growing but
even after that due to metamorphic pressure.
When a gem breaks in a direction which is parallel to the twinning
plane, it is termed as parting.
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| CONTACT TWINNING |
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| CYCLIC TWINNING |
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| LAMELLAR TWINNING |
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| PENETRATION TWINNING |
Fracture: besides cleavage and parting, any other break in a gem is
called a fracture. Most transparent gems break in a typical pattern creating circular and concave ridges. This is called conchoidal fracture. There are other kinds of fractures as well splintery, granular, sub-conchoidal etc.
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| CONCHOIDAL FRACTURE |
Specific Gravity: it is the measurement of density of a mineral. Two
minerals of the same size might weigh differently due to the variation in the density of minerals. Specific gravity
can be described as a ratio between the weight of the mineral in air to the difference of the weight of mineral in air and mineral in water.
In other words SG= wt. of mineral in air/ wt. of mineral in air – wt.
of mineral in water.
SG is a very important tool in gem identification.
Properties of gemstones is an imperative subject of gemology. It is a vital aspect which not only aids one
in understanding the very nature of gems but also is of significant help to
gemstone facet-ors. Along with these, it also gives
crucial hints which can help one take good care of their gems.
*The Author is a Colored Stone Graduate from Gemological Institute of America, Mumbai and a Diamond Graduate from Solitaire Diamond Institute, Bangalore. She also holds a Diploma in Jewelry Designing & Manufacturing from Jewelry Product Development Centre, Jaipur. She also blogs at 'The Jewel Affair'.
*Picture Courtesy: Google Images
