| The
luster of a gemstone is the amount of light reflected from its surface. Most loose gemstone have
a glass-like (vitreous) luster. The amount that light rays are bent by a gemstone is its
refractive index (RI) which is measured using a refractometer. A gemstone's birefringence is
the difference between the minimum and maximum RI. Gems are generally used
to create jewelry and enhance the beauty of objects. They are also collected to be
privately admired.
Gems must be relatively hard and fairly
resistant to chemical alteration to be able to endure wear and tear. Hardness and
durability is one of the key characteristics used to define a gemstone. The scale of 1 to 10
used to determine gem hardness was created by German mineralogist Friedrich Mohs. Diamond
(10) is the hardest and talc (1) the softest. Generally, any gemstone with a hardness less
than quartz (7) is considered lacking the durability appropriate for jewelry. Steel is
about 6.5, glass is about 5.5, and a finger or toe nail is about 2.5.
| Toughness and hardness are different
characteristics of a gemstone. Diamonds is the hardest natural occurring material there is but
is not very tough. It's weak atomic bonding along cleavage planes means that, despite its
hardness, it can break if it collides with a hard material. Jadeite has hardness of only 7
but is comprised of interlocking fibers, making it the toughest gem. |
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Fibers or fibrous cavities within
a gemstone such as a sapphire reflect the light which creates a star within the stone. A
six-ray star has three sets of parallel fibers. Sometimes cutters can create a 12-ray star
sapphire.
A break in a natural or imitation gem along
a surface that is not a cleavage plane is called a fracture. There are three types of
fractures: uneven, hackly, and conchoidal. The conchoidal (shell like) fractures of
colored glass is one way to determine a gem is an imitation.
The value of a gem is determined by type,
color, clarity, size, rarity, popularity and weight. Gems are measured in carats (one
fifth of one gram).
Most gemstone have a crystalline structure. Crystals have planes of symmetry and are divided into
seven symmetry systems. A plane of symmetry causes a crystal to have a mirror image when
an imaginary axis is drawn through it. The number of axes, their length, their angle to
each other determine the system to which a crystal belongs.
Sapphire is classified as trigonal
because it has three planes of symmetry and four axes. Three axes are at 60 degrees to
each other in the same plane. The fourth axis is perpendicular and unequal in length to
the other three. Spinel is classified as isometric because it has three axes, equal in
length, and at right angles to each other.
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The flat surfaces
of well formed crystals are called faces. The internal atomic structure of a crystal
determines the faces and angles between them. A massive crystal is one comprised of such
small, poorly formed crystals that the faces cannot be distinguished. A crystal's habit is
the defined sets |
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and their angles which create its usual shape. |
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A gem's color
is produced when light passes through it. Some colors of the spectrum are absorbed within
the crystal structure. Those that emerge combine to give the gem its color. If all the
light emerges it will be colorless. This absorption spectrum is used to distinguish
between similarly colored but different gemstone and is measured using a spectroscope. A
spectroscope could be needed to tell the difference between a ruby colored by chromium and
a very similar looking almandine colored by iron.
A gemstone's ability to split white light into
the colors of the spectrum is called its dispersion. A gem's dispersion is the difference
between its RI for a red ray, the least refracted color, and its RI for a blue ray, the
most refracted.
Sometimes it is necessary to use color
filters, ultraviolet light, x-rays, or measurements of magnetism to accurately identify
gemstone because the color of different gems can be very similar to the naked eye and color
can be changed by oiling, staining, irradiation, or heat treatment.
Light temperature
is inconsistent. An actual gemstone looks different depending on time of day,
geographical
location, weather and lighting conditions. A gemstone you see on a clear day at 10 am in
Bangkok will look different at the same time on a similar day in New York.
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Arnold with Locals in Madagascar, mining for Blue & Pink Sapphire |
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