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.