Rarely can be viewed with the naked eye. These are any of the impurities that are on the surface of the diamond. External flaws are the result of wear and tear on the diamond or from damage caused during cutting. These are any of the many impurities that reside solely in the interior of the diamond. There are many kinds of internal flaws.
Clouds are microscopic inclusions that lower transparency.
Carbon or other dark spots. The majority of imperfections in diamonds are the result of breakage or fractures, in the diamond chemical structure. The highest clarity grade given. The diamond in question will be without imperfections visible with x10 magnification. The better they are the higher the price per carat.