Most diamonds develop natural "inclusions" (crystals, feathers, etc.) during their formation deep within the earth.

Diamond  clarity is determined by the number and/or nature of these inclusions. The fewer the inclusions, the rarer the diamond and the greater the value. Flawless and internally flawless diamonds are exceedingly rare. The term "flawless" is a highly restricted one. Under Federal Trade Commission  rules a diamond  can only be deemed flawless when no internal or external imperfections are visible to a professional eye under 10 power magnification using a binocular microscope in good light. While inclusions may not affect the beauty of a diamond, they do affect its value and price.