As with diamonds, gemstone value and quality are evaluated according to the "Four Cs": color, clarity, cut, and carat weight.

There's a fifth, much less scientific factor--perception.

COLOR

• Not surprisingly, color is the key factor with colored gemstones.
• A common misperception in judging gems is that people assume the darker the color, the better the stone. This isn't true: color can be too dark, as with some sapphires that look more black than blue. If a gem's color is overly dark, it tends to be subdued and lifeless.
• A much better rule of thumb is the brighter, richer and more vivid the color, the better. Within each gemstone variety it is the clear, medium-tone, very intense and saturated basic color that is most preferred. Muted colors or colors between hues, which some might find very attractive, are usually less expensive.
• Always remember to look at the color in different kinds of light, since the light spectrum can affect gem color greatly.

CLARITY

• A good cut, while it may not cost more, can add or subtract quite a lot of beauty to a stone.
• A well-cut, faceted gemstone reflects light 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.

CARAT WEIGHT

• Gemstones are generally sold by weight rather than size.
• Some gems are denser than others, so the same weight stone may be a different size!
• The carat weight will also affect the price. Large gemstones are usually rare, marking up the price per carat of the stone.

PERCEPTION

• For every gemstone, pricing follows common sense: the better the final visual effect of all the quality factors of the stone, the more valuable that stone is. But different varieties of gemstones have different price ranges. This is where the perception factor comes in.
• Some varieties are lower in price because they are readily available; some, because the color isn't very popular (brown and yellow stones, for example); some, because the material is relatively soft; and some, because they have all the right stuff but no one knows it.
• There are plenty of examples of beautiful, rare gemstones costing less than gems that are not so rare -- merely because they possess a funny name, or people confuse them with an inexpensive variety, or they are of a gem variety few of us are acquainted with.
• In the past, the gemstone business was dominated by the big three: ruby, emerald and sapphire. These often are called precious stones, a term probably derived from the French label, "pierres precieuse."
• When other gemstone varieties began to gain popularity, the jewelry industry got tired of calling them "colored gemstones other than ruby, emerald and sapphire." Unfortunately, the term they made up for these other stones was "semi-precious." Too bad the French terminology was not followed in this instance, or these other stones would have been known as "fine gemstones," which is a much better and more accurate term.
• The problem with "semi-precious," and the reason the jewelry industry essentially banned its use, is that it's a misnomer. Rubies, emeralds, and sapphires can sell for less than $100 per carat, while a fine Paraiba tourmaline, for example, can sell for $20,000 per carat!.