Diamond color is all about what you can't see. Diamonds are valued by how closely they approach colorlessness - the less color, the higher their value. (The exception to this is fancy colored diamonds, such as pinks and blues, which lie outside this color range.) Most diamonds used in jewelry are nearly colorless with tints or yellow or brown.
Color grades are determined by comparing each diamond to a master set. Each letter grade represents a range of color and is a measure of how noticeable a color appears.
Clarity is determined by the number and size of internal characteristics (called inclusions) and external characteristics (called blemishes), which both occur naturally in diamonds. Small inclusions that are visible only under magnification are not likely to diminish the beauty of a diamond. The smaller the inclusion, the less likely that it will interfere with light as it passes through the diamond.
A polished diamond's proportions, or "cut" affect its light performance, which in turn affects its beauty and overall appeal. Diamonds with fine proportions, symmetry, and polish optimize their interaction with light and have increased brightness, fire, and scintillation.
The weight of a diamond is measured in carats. It is the easiest of the four characteristics to determine. A carat is 200 milligrams (1/5 of a gram) and is divided into 100 points (like pennies to a dollar). Thus a half carat stone is a diamond of 50 points and is listed as a 0.50 carat.