Charles Milner, a Chicago-based wholesaler, says consumers should consider "the four 'C's" -- color, clarity, cut and carat weight -- when shopping for diamonds. Here's what Milner suggests looking for in each category:

Color -- The less color, the better, so colorless (sometimes called blue-white) is best. Ask your jeweler for the color "grade" of the stone.

The Gemological Institute of America has established these color grades for jewelers to use in evaluating diamonds:

D,E and F : The highest grades, indicating colorless;

G,H and I : Indicate slight, but increasingly higher amounts of color;

F: "The average consumer would start seeing yellow in the stone, according to Milner.

Clarity -- An unclear stone lacks brilliance.

You can ask to see the stone's clarity rating based on Genological Institute of America grades:

FL: Means the stone is flawless and therefore has no blemishes;

VVS: Means very, very slightly included (gem terminology for cracked or blemished), and the decline in brilliance is probably not visible to the untrained eye;

VS; Stands for very slightly included, and the reduced brillance can be observed by a consumer who compares the stone to a flawless stone;

SL: Indicates the stone is slightly included, and its lack of brilliance should be noticeable to consumers who have made some comparisons.

The errors to look for, either with the naked eye or through a jeweler's loupe (a small magnifying glass) are "black and white cracks or speckles that look like salt and pepper," according to Milner.

Cut -- The more cuts (facets) in the diamond, the more light it refracts and the more brilliant it appears.

Some of the types of cut include round, brilliant, radiant and marquis. Careful inspection will show uniformity of cut, whether or not some of the stone's angles are shallower or deeper than others. Also, if the top (or table) of the stone is cut too broadly, the stone may look imbalanced.

Carat -- This measures the weight of the stone (one carat equals 1/142 of an ounce).

Generally, the higher the carat weight, the higher the price tag. Carat differences are generally obvious to the average eye after a little comparative shopping. Certain cuts may make stones look larger or smaller than their carat weight indicates.