The four characteristics: cut, carat (weight), clarity, and color determine how much a diamond is worth.
Diamond proportion is very important. Round brilliant diamonds are usually cut with 58 facets. The proportionional similarity of these facets on the diamond affect how much light will be reflected to the viewer. The better the cut, the more the diamond will sparkle. Ask for the table percentage. A good table percentage is between 55-60%. Cut also determines the shape such as pear, round or oval. When having a diamond mounted, appraise and record the stone's measurements which never change. After the stone is mounted, verify that it matches the measurements and/or appraisal certificate.
Carat is the measurement of a gem's weight. Larger gems often cost more per carat because of their size. There are 100 points to a carat. A 50 point diamond equals 1/2 a carat. One gram is 5 carats. Ascertain the actual point size of a diamond instead of the fractional weight. Some jewelers will try to sell a .90 diamond as a 1 carat diamond which would be much more expensive.
Clarity or clearness ranges from flawless (perfect) to I (included).
Colors range from D - X for white and yellow diamonds. D is the whitest. At S they are "Fancy" yellow Diamonds. There are also, pink, red, green, blue and brown diamonds which are typically irradiated.
Always ask for a guarantee of the color and clarity, as some states permit dealers to be off on color or clarity by one grade.
Diamonds are sometimes filled for clarity, and irradiated or painted for color. Diamonds with inclusions can be filled with glass to make them seem clearer. This does not fix the inclusion, just covers it. A filler can be damaged by heat, ultrasonic cleaning, and by re-tipping. Look at the filled diamond closely and rotate it under light, you should be able to notice a bluish flash.
Diamonds that are irradiated for color may be affected by heat. Paint on a diamond to offset a yellow tinge wears off rather quickly. If the diamond is represented as not being treated, get a notarized statement from your jeweler saying so. Many states require dealers to tell you about these treatments by law.
Diamonds are commonly believed to be unbreakable, which is not the case. Don't wear diamonds when doing heavy activities and always protect them from scratches and sharp blows. To avoid scratching, keep diamonds stored separately from each other and other jewelry. The best way to clean a diamond is to use a jeweler's polishing cloth.