Rare in this case means very, very expensive. Many stores here in the US also use milky diamonds. Clarity Clarity means absence of inclusions. Diamonds may show tiny traces of other minerals trapped during the crystallization process. They typically show up as minute black or white marks.
Light passes most optimally through a diamond that has no inclusions.
Nature makes nothing absolutely pure and perfect. There are, however, diamonds that at 10x magnification show no inclusions. At the other end of the scale are diamonds that show impurities that are easily visible to the naked eye. The scale used simply ranges from the most difficult to the easiest to see at 10x magnification. The clarity of most diamonds available on the market falls between these two extreme values.