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 . Structural irregularities, included crystals, or knots produce strains that can cause breaks. Most diamond professionals use the term feather for any break in a diamond. Leaving part of the crystal's original surface on the cut stone produces a natural .