The four Cs of diamonds are color, cut, clarity and carat weight. According to jeweler Harry Winston's printed diamond guide, here are things to keep in mind when considering each factor:
Color. The Gemological Institute of America grades diamonds on a color scale of D through Z, with D being colorless and the most desirable. Moving down the scale, diamonds tend to become more yellow. The ratings do not, however, apply to fancy colored diamonds. Rare yellow, pink and blue diamonds are among the most sought after stones.
Cut. Diamonds -- whether they're big or small -- need to be cut to the proper proportions to allow light to reflect inside. Stones cut too deep or too shallow will have poor brilliance.
Clarity. Typically, the greater the number and size of the flaws, the lower the clarity grade and the less rare the diamond. The best rating is FL, for flawless, which means the diamond is without any of "nature's birthmarks," called inclusions. The lowest rating is "I," noting that imperfections such as crystals, clouds and feathers are visible or "included."
Carat weight. Each carat is equal to 100 points. Larger diamonds are more valuable because they're found less frequently in nature, but diamonds of equal size vary in value depending on their brilliance.