Virtually all natural diamonds contain identifying characteristics, many of which are invisible to the naked eye. Clarity is the degree to which a stone is free from marks called blemishes and internal features called inclusions .
Inclusions normally have a greater impact on grade, value, beauty, and durability than do blemishes.
Irregularities in the atomic arrangement of a growing diamond crystal can cause what is known as graining . Graining may look like faint lines in the stone. Many diamonds contain crystals of other minerals. Tiny ones are called pinpoints . Included diamond crystals that reach the surface are called knots .