A gemstone or gem (also called a precious or semi-precious
stone, a fine gem, or jewel) is a piece of mineral, which, in cut and polished
form, is used to make jewelry or other adornments.[1][2] However certain rocks,
(such as lapis lazuli) and organic materials (such as amber or jet) are not
minerals, but are still used for jewelry, and are therefore often considered to
be gemstones as well. Most gemstones are hard, but some soft minerals are used
in jewelry because of their luster or other physical properties that have
aesthetic value. Rarity is another characteristic that lends value to a
gemstone. Apart from jewelry, from earliest antiquity until the 19th century
engraved gems and hardstone carvings such as cups were major luxury art forms;
the carvings of Carl Fabergé were the last significant works in this tradition.
Characteristics and classification
The traditional classification in the West, which goes back
to the Ancient Greeks, begins with a distinction between precious and
semi-precious stones; similar distinctions are made in other cultures. In
modern usage the precious stones are diamond, ruby, sapphire and emerald, with
all other gemstones being semi-precious.[3] This distinction reflects the
rarity of the respective stones in ancient times, as well as their quality: all
are translucent with fine color in their purest forms, except for the colorless
diamond, and very hard,[4] with hardnesses of 8–10 on the Mohs scale. Other
stones are classified by their color, translucency and hardness. The
traditional distinction does not necessarily reflect modern values, for
example, while garnets are relatively inexpensive, a green garnet called
Tsavorite, can be far more valuable than a mid-quality emerald.[5] Another
unscientific term for semi-precious gemstones used in art history and
archaeology is hardstone. Use of the terms 'precious' and 'semi-precious' in a
commercial context is, arguably, misleading in that it deceptively implies
certain stones are intrinsically more valuable than others, which is not the
case.
In modern times gemstones are identified by gemologists, who
describe gems and their characteristics using technical terminology specific to
the field of gemology. The first characteristic a gemologist uses to identify a
gemstone is its chemical composition. For example, diamonds are made of carbon
(C) and rubies of aluminium oxide (Al2O3). Next, many gems are crystals which
are classified by their crystal system such as cubic or trigonal or monoclinic.
Another term used is habit, the form the gem is usually found in. For example
diamonds, which have a cubic crystal system, are often found as octahedrons.
Gemstones are classified into different groups, species, and
varieties. For example, ruby is the red variety of the species corundum, while
any other color of corundum is considered sapphire. Emerald (green), aquamarine
(blue), red beryl (red), goshenite (colorless), heliodor (yellow), and
morganite (pink) are all varieties of the mineral species beryl.
Gems are characterized in terms of refractive
index, dispersion, specific gravity, hardness, cleavage, fracture, and luster.
They may exhibit pleochroism or double refraction. They may have luminescence
and a distinctive absorption spectrum.
Value
There is no universally accepted grading system for
gemstones. Diamonds are graded using a system developed by the Gemological
Institute of America (GIA) in the early 1950s. Historically, all gemstones were
graded using the naked eye. The GIA system included a major innovation: the
introduction of 10x magnification as the standard for grading clarity. Other gemstones
are still graded using the naked eye
Gems are characterized in terms of refractive index, dispersion, specific gravity, hardness, cleavage, fracture, and luster. They may exhibit pleochroism or double refraction. They may have luminescence and a distinctive absorption spectrum.
A mnemonic device, the "four Cs" (color, cut,
clarity and carats), has been introduced to help the consumer understand the
factors used to grade a diamond With modification, these categories can be
useful in understanding the grading of all gemstones. The four criteria carry
different weight depending upon whether they are applied to colored gemstones
or to colorless diamond. In diamonds, cut is the primary determinant of value,
followed by clarity and color. Diamonds are meant to sparkle, to break down
light into its constituent rainbow colors (dispersion), chop it up into bright
little pieces (scintillation), and deliver it to the eye (brilliance). In its
rough crystalline form, a diamond will do none of these things; it requires
proper fashioning and this is called "cut". In gemstones that have
color, including colored diamonds, it is the purity and beauty of that color
that is the primary determinant of quality
Aside from the diamond, the ruby, sapphire, emerald, pearl
(strictly speaking not a gemstone) and opal have also been considered to be
precious. Up to the discoveries of bulk amethyst in Brazil in the 19th century,
amethyst was considered a precious stone as well, going back to ancient Greece.
Even in the last century certain stones such as aquamarine, peridot and cat's
eye have been popular and hence been regarded as precious.
Nowadays such a distinction is no longer made by the
trade.[12] Many gemstones are used in even the most expensive jewelry,
depending on the brand name of the designer, fashion trends, market supply,
treatments, etc. Nevertheless, diamonds, rubies, sapphires and emeralds still
have a reputation that exceeds those of other gemstones.
Grading:
Each laboratory has its own methodology to evaluate
gemstones. A stone can be called "pink" by one lab while another lab
calls it "Padparadscha". One lab can conclude a stone is untreated,
while another lab might conclude that it is heat-treated.[12] To minimise such
differences, seven of the most respected labs, AGTA-GTL (New York), CISGEM
(Milano), GAAJ-ZENHOKYO (Tokyo), GIA (Carlsbad), GIT (Bangkok), Gübelin
(Lucerne) and SSEF (Basel), have established the Laboratory Manual
Harmonisation Committee (LMHC), for the standardization of wording reports,
promotion of certain analytical methods and interpretation of results. Country
of origin has sometimes been difficult to determine, due to the constant
discovery of new source locations. Determining a "country of origin"
is much more difficult than determining other aspects of a gem
Cutting and polishing
A few gemstones are used as gems in the crystal or other
form in which they are found. Most however, are cut and polished for usage as
jewelry. The picture to the left is of a rural, commercial cutting operation in
Thailand. This small factory cuts thousands of carats of sapphire annually. The
two main classifications are stones cut as smooth, dome shaped stones called
cabochons, and stones which are cut with a faceting machine by polishing small
flat windows called facets at regular intervals at exact angles.
Stones which are opaque such as opal, turquoise, variscite,
etc. are commonly cut as cabochons. These gems are designed to show the stone's
color or surface properties as in opal and star sapphires. Grinding wheels and
polishing agents are used to grind, shape and polish the smooth dome shape of
the stones
Gems which are transparent are normally faceted, a method
which shows the optical properties of the stone's interior to its best
advantage by maximizing reflected light which is perceived by the viewer as
sparkle. There are many commonly used shapes for faceted stones. The facets
must be cut at the proper angles, which varies depending on the optical
properties of the gem. If the angles are too steep or too shallow, the light
will pass through and not be reflected back toward the viewer. The faceting
machine is used to hold the stone onto a flat lap for cutting and polishing the
flat facets Rarely, some cutters use special curved laps to
cut and polish curved facets.
Color:
Color
of any material is due to the nature of light itself. Daylight, often called
white light, is actually all of the colors of the spectrum combined. When light
strikes a material, most of the light is absorbed while a smaller amount of a
particular frequency or wavelength is reflected. The part that is reflected
reaches the eye as as the perceived color. A ruby appears red because it
absorbs all the other colors of white light (green and blue), while reflecting red
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