Most diamonds have birthmarks. Depending on their position within the gem, some are obvious but most are unimportant. They come in various forms- tiny carbon specks, little feather-like lines, minute bubbles and so on. In many cases they aren't visible to the naked eye. The flawless diamond, one in which no blemish of any kind can be seen even under 10-power magnification, is obviously the most sought-after stone. But these are rare indeed and priced accordingly. Just as there is a Gemological Institute of America color scale, so is there one for clarity. It starts, naturally enough, with flawless and then moves down through "very very slightly included" to "slightly included" to "imperfect."