Colored gemstones

Gems come in an infinite combination of pure "spectral" colors (like the colors of the rainbow) with varying degrees of black, brown, gray or white.

In nature, a gemstone with pure color is very rare. Generally, the closer a gem is to a pure spectral color, the more rare and valuable it is. The way a gem is cut affects its intensity and tone.

Very light and very dark stones are usually worth less than stones of an even, moderate hue. While dark gems may be generally prettier than pale, watery gems, those with too much black, gray or brown generally have less appeal.
 

Describing the color

Hue: the body color, according to the visual spectrum.

Intensity: the brightness of the colors.
Tone: the darkness or lightness of the overall color, which is affected by how much gray, brown, black or white there is.
Distribution: the evenness of the colors.
 

The cut makes the diamond

Remember the four C's of diamond quality? Alphabetically, they are: carat weight, clarity, color and cut. A properly-cut diamond will dazzle for a lifetime. There are 58 facets in a diamond with a "modern brilliant" cut. This particular cut is often used with other gemstones as well.
 

Parts of a diamond

The two main parts of a brilliant-cut diamond are the crown and the pavilion. The flat part on top is called the table. Between the crown and pavilion facets is the outer rim, called the girdle. The lowest part of the diamond is called a culet. It appears to be a point, but when the stone is cut properly, the culet is actually a facet.

 
 

Proportion is the key

When the crown is about one-third the depth of the pavilion (that is, the distance from the girdle to the culet) the proportion is more than likely to be acceptable. Variations from this ideal tend to result in a less brilliant diamond. For example, if a stone is cut too deep or too shallow, the gem appears dark or dull in the center because the facets do not reflect light properly (back to the viewer's eye).

You don't have to be an expert to be able to see whether or not a diamond has been properly cut. If it's dazzling, chances are the cut and proportion are very close to the ideal.