The Gemological Institute of America ( GIA ) grades many diamonds of one carat or greater according to the four Cs: cut, color, clarity, and carat weight. The interplay of the four Cs determines a diamond's value.

The proper cut enhances a diamond's capacity to reflect light and makes it sparkle more; a diamond's depth, and the distance from top to bottom, should be about 58-61 percent of its width, which means the gem should be wider than it is tall.

Most diamonds contain slight traces of yellow or brown, and the more colorless a diamond, the greater its value; most of the diamonds that retailers sell are in the near-colorless color grades, or G through J.

Clarity refers to the number of spots, or inclusions, in a diamond, and the fewer the inclusions, the clearer the diamond and the brighter its sparkle; retailers mostly sell diamonds in the very slightly included clarity grades and in the slightly included clarity grades.

A diamond's value rises as its size, or carat weight, increases; a carat is one-fifth of a gram, or about 7 / 1,000 of an ounce.

A definition of the "Four C's"  

Carat - This is the unit of weight used for diamonds. The word Carat derived from carob seeds used to balance scales in ancient times.

A carat is equal to 200 milligrams and there are 142 carats to an ounce. Carats are further subdivided into points. There are 100 points to a carat. For example, a 45-point diamond weighs a little less than half a carat. Because larger diamonds are quite rare, they have a greater value per carat.

Color - Although a diamond may be any color of the spectrum, grading a cut stone for color means deciding the amount by which it deviates from white (colorless). Completely colorless, icy-white diamonds are rare, and therefore, more valuable. The best way to see the true color of a diamond is looking at it against a white surface. Although most diamonds are a shade of white, they do come in all colors - pale yellow, canary, pink, red, green, blue and brown. These are called "fancies," and they are valued for their depth of color, just as white diamonds are valued for their lack of color.

Clarity - A diamond's clarity is determined by taking into account the number, size, placement, color and nature of any internal "inclusions" or external surface irregularities. Inclusions are a diamonds fingerprint - imperfections such as spots, bubbles or lines - included in the stone when it was crystallized from carbon millions of years ago. These marks make each stone unique, for no two diamonds have the same inclusions in the same places. When inclusions do not interfere with the passage of light through the stone, they do not affect its beauty. However, the fewer the inclusions, the more valuable the diamond. Under Federal Trade Commission rules, a diamond can be called "flawless" only when no imperfections are visible to a trained eye under 10 power magnification and in good light. Gemological Institute of America's (GIA) quality analysis system is the most widely used for grading gemstones in the United States. Clarity is graded according to the relative position of the diamonds on the Flawless-to-Imperfect scale.

Cut - Diamonds are cut according to an exact mathematical formula. A finished diamond has 58 "facets," which are the small, flat polished planes cut into a diamond, so that the maximum amount of light is reflected back to the viewer's eye. This reflection is called "brilliance," and is extremely important in evaluating the quality of a diamond. The widest circumference of a diamond is the "girdle." Above the girdle are 32 facets plus the "table," the largest and topmost facet. Below the girdle there are 24 facets plus the "culet," or point. Cut also deals with the shape of the diamond. Traditional shapes are round, emerald, marquise, pear, oval and heart.