The Four C's: Cut, Color, Clarity, and Carat-Weight.

Cut

Most round, brilliant-cut or fancy-shaped diamonds possess 58 carefully angled flat surfaces, called facets, whose placement will affect the fire, brilliance and ultimate beauty of your diamond. It is the skill and imagination of the craftsman's effort in the Diamond cutting process, that reflects the maximum amount of light back to the eye.

Color

The most prized diamonds are colorless diamonds, because their beauty depends entirely upon their remarkable optical properties. In such diamonds, all the colors of the rainbow are reflected back to your eye. While the majority of gem diamonds appear to be colorless, others can contain increasing shades of yellow to brown, some of which are referred to as champagne diamonds. Other diamonds of exceptional color--red, blue, green, pink, and amber--are known as "Fancies."

The color grading scale varies from totally colorless to light color or tinted. The difference between one grade and its neighbor is very subtle. Experts use master diamonds of known color for comparison.

The lowest quality of diamonds are yellow in color.

Slightly better are light yellow.

Better are very light yellow.

Good quality are faint yellow.

Very good quality are near colorless. 

The excellent are completely colorless.

Clarity

Diamonds because of their unique optical properties, are capable of producing the maximum amount of brilliance. Minute crystals of diamond or other minerals are contained in almost all diamonds, a diamond that is virtually free of inclusions and surface markings will be judged as flawless. In these type of flawless diamonds, inclusions do not interfere with the passage of light or spoil the beauty. But these diamonds are expensive and extremely rare

To determine a diamond's clarity grading, it must be examined under a 10x magnification by a trained, skilled eye. Without high magnification, you may never see these inclusions. However, the fewer there are, the rarer your diamond will be.

The GIA scale uses abbreviations. This is what they stand for:

"FL" stands for "Flawless"

Also used is "IF" which stands for " Internally Flawless."(no inclusions visible under 10X magnification)

"V V S" stands for "Very very slightly Imperfect." (inclusions are minute and difficult to locate under 10X magnification)

"V S" stands for "Very slightly imperfect." (inclusions are minor and hard to detect under 10X magnification)

"S I" stands for "Slightly Imperfect." (inclusions are noticeable and relatively easy to locate under 10X magnification)

"I" stands for "Imperfect." (inclusions are obvious under 10X magnification and are visible to the naked eye)

Carat-Weight

As with all precious stones, the weight--and therefore the size--of a diamond is expressed in carats. One carat is divided into 100 "points" so that a diamond of 25 points is described as a quarter of a carat or 0.25 carats. Size is the most obvious factor in determining the value of a diamond, but now you know that two equal sizes can have very unequal prices depending on their quality. However, remember that diamonds of high quality can be found in all size ranges.