The Basics
As with diamonds, gemstone quality and value are evaluated according
to the "Four Cs": color, clarity,
cut, and carat weight. And there's one other, much less scientific factor--perception.
Color, not surprisingly, is the key factor. A common misperception in
judging gems is people assume that the darker the color, the better
the stone. That isn't true: color can be too dark, as with some sapphires
that look more black than blue. If a gem's color is too dark, it is
subdued and lifeless. A much better rule of thumb is the brighter and
more rich and vivid the color the better. In general, within each gemstone
variety, a clear, medium-tone, very intense and saturated basic color
is the most preferred. Muted colors or colors between hues, which some
might find very attractive, are usually less expensive. Look at the
color in different kinds of light, since the light spectrum can affect
color greatly.
The next most important factor affecting value is clarity, 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. Inclusions can also act as a birthmark, telling
where a particular gemstone was mined.
A good cut is something that may not cost more but can add or subtract
a lot of beauty. A well-cut faceted gemstone reflects light back evenly
across its surface area when held face up. If the stone is too deep
and narrow, areas will be dark. If it is too shallow and wide, parts
of the stone will be washed out and lifeless.
Gemstones are generally sold by weight not by size. Some gems are denser
than others so the same weight stone may be a different size! The carat
weight also affects the price--large gemstones are usually more rare,
so the price per carat is higher.
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