Little girls often dream about weddings and brides-to-be show off the engagement ring. But rather than the setting or the shape, it is the diamond’s cut, clarity and color, along with size (measured in carats), that are the most important factors in determining worth. Combined, these four factors -- cut, clarity, color and carats -- determine a diamond's value. Diamonds are rated, or graded by experts, most often by those certified by the Gemological Institute of America (GIA)
Diamond color ranges from D (colorless) to Z (light yellow in color). Colorless diamonds allow more light and so show more sparkle and fire than other diamonds. D through F are the most valuable ratings, but I, which is in the near-colorless range, is still considered a good color rating
The clarity of a diamond is a measure of the diamond’s faults, known as inclusions. Clarity ranges from F for flawless (an exceptionally rare diamond) to I1 and I2 for diamonds that have visible flaws. S1 (Slightly Included) is a good clarity rating, with VS2 (Very Slightly Included) being very good and VS1 considered excellent.
The cut of a diamond is another important factor in determining worth. A poorly cut diamond loses its brilliance.