It is a rarity. Most inclusions are not visible to the naked eye, but only under powerful magnification. The fewer and smaller the inclusions, the more rare and beautiful the diamond. While inclusions usually cannot be seen, their presence reduces the price because the diamonds are more readily available.
Clarity is graded on a scale with a range from internally flawless to imperfect.
Usually, an unaided eye will begin to see inclusions at I1 and below. Mervis Diamond Importers offers all clarity grades, with an emphasis at the upper-end of the scale.