The basics of colour gems valuation
Colour gemstones quality and value are evaluated according to the 4 C´s: Color, Clarity, Cut and Carat weight.
Common Sense
In general, gemstone pricing within each variety follows common sense: the
better final visual effect of all the quality factors, the more valuable it is.
But different varieties have different price ranges. Some varieties are lower
in price because they are readily available, some because the color isn´t
very popular, some because the material is relatively soft, and some because no
one knows it.
There are plenty of examples of beautiful rare gemstones that cost less than gems
that are less rare because they have a funny name, or people get them confused with an
inexpensive variety or no one has ever heard of them.
The Colour
A common misperception in judging gems is that people assume that the darker
the colour, the better the stone. This isn´t true: colour can be too dark,
as with some sapphires that look more black than blue. If a gem´s colour is
too dark, it is subdued and lifeless. A much better rule of thumb is the brighter
and more rich and vivid the colour the better. In general, within each gemstone
variety, a clear, medium-tone, very intense and saturated basic colour is the most
preferred.
Gemological Institute of America (GIA) suggests to describe color for colored
gemstones as Hue, Tone and Saturation, where Hue describes the dominant and any
additional colors visible in a stone Tone: the relative lightness or darkness of
a hue or color sensation. Saturation: the strength, purity or intensity of the
hue present in a color sensation. For more infromation, please refer to Describing Color in
Gemstones, Color Communication.
Look at the colour in different kinds of light, since the light spectrum
can have a dramatic effect on the appearance of any colored stone.
Incandescent lighting is rich in red, orange and yellow wavelengths and thus
pushes an object´s color in that direction. In contrast, skylight
is more balanced, pushing the color in the opposite direction.
The color of the background against which a gem is examined also can have a
major effect on color. The yellow background color counters the bluish tint
commonly present in ruby, making the gems appear more red. Yellow cellophane-lined
stone papers or brass tweezers serve the same purpose. For judging color, a plain
white background is best.
The Clarity
When we speak of a gemstone clarity, we are referring to the
presence of identifying characteristics on and within the stone.
While most of these characteristics are inherent qualities of the
rough material and have been present since the earliest stages of
the crystal´s growth below ground, a few are actually a result
of the harsh stress that a stone undergoes during the cutting process
itself
Gemstone clarity is the next most important factor affecting value,
with clear transparent gemstones with no visible flaws being the most
valued. Some gemstone varieties, notably emerald and red tourmaline,
are very rare without inclusions of some kind so the price structure
takes this into account. Other gems are actually valued for their
inclusions. Tiny inclusions reflecting back light put the eye in
cat´s-eye chrysoberyl and the star in star sapphire and ruby.
There is no universally accepted standard for grading the clarity
of colored gemstones. Although the GIA colored stone clarity grading
system is a notable attempt to address the issue, it is still the subject
of a great deal of discussion and debate.
The Cut
First, do not confuse cut with shape. Shape refers to the general
outward appearance of the stone, (such as round, emerald, or pear).
The cut is what gives a gemstone its beauty and brilliance. The most
usual method of fashioning a gem is to cut the surface into a number
of flat faces, known as facets. Polishing is the oldest form of fashioning.
An ideal cut which reflects all the light in an even pattern without
any darkness or windowing is usually preferable. In faceted stones,
specific indices and critical angles are what produce the maximum brilliance.
The density or intensity of a color is always an important consideration for
cutters. Any gemstone will become lighter as its size is reduced. The color of
over-dark stones will improve as their size is reduced while light stones will
become even lighter and lose some of their value as a result of cutting. Because
of their beauty and rarity, gemstones are valuable, especially rare stones with
good color and clarity. Well-cut and proportioned stones look better and are
easier to sell. As dealers and cutters we frequently re-cut poorly faceted
stones, using our experience to select those with color and clarity and then
drawing out their hidden potential, while still maximizing final weight and value.
The Carat Weight
A carat is a unit of measurement equal to 200 milligrams or 0.2 grams.
The word "carat" is taken from the carob seeds that people
once used in ancient times to balance scales. So uniform in shape and
weight are these little seeds that even today«s sophisticated
instruments cannot detect more than three one-thousands of a difference
between them.
Gemstones are generally sold by carat weight? not by size (calibrated stones
are exemption). Carat weights do not determine size. The density of gems varies
from very light stones like opal to especially heavy stones like sphalerite
or zircon.
Here are some useful formulas for estimating the weight of stones in
settings, where the stones cannot be removed for accurate weighing. All
weights are given in carats, and all measurements in millimetres.
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| Round and oval shaped faceted stones |
Weight = average diameter * total depth * S.G. * .0018 |
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| Rectangular shaped faceted stones: |
Weight = length * width * total depth * S.G. * .0026 |
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| Square or emerald shaped faceted stones: |
Weight = length * width * total depth * S.G. * .0025 |
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| Pear or heart shaped faceted stones: |
Weight = length * width * total depth * S.G. * .0018 |
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Larger stones are always rarer and per-carat prices will usually be more than
equivalent to similar stones in a smaller size. Relationships between per-carat
prices and sizes will vary with each specific gemstone group. As a starting point,
it is possible to use the following guidelines:
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| 1/2 cts to 1cts |
Price($x/cts) |
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| 1 cts to 2cts |
Price($2x/cts) |
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| (2 cts to 3cts |
Price($4x/cts) |
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| 3 cts+ |
Price($4-5x/cts) |
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Market Factors
One of the great mysteries for the gem novice is the relationship between
price and quality. In a perfect world, price would directly relate to
quality, weight and rarity. Unfortunately, market factors can have as much,
or even greater, impact on prices as does quality.
Market supply vs. demand, when stones which are plentiful and/or in low demand
will be cheaper than those which are rare and/or in high demand. Financial
situation of the seller, where sellers who need money will obviously be more
flexible on price. Similarly, those who are not in need are less willing to
reduce their price. Prices can vary dramatically depending on the seller´s
overhead. And finally, buyer and seller personal relationship. When the buyer
and seller enjoy each other´s company, they often make special provisions
for one another.
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