Diamonds can have tiny crystals and minerals imbedded in them, even other diamonds.  These internal characteristics are usually referred to as inclusions.  Many inclusions cannot be seen without magnification, though a large chunk or cluster of crystals that detract from a diamond’s appearance can lower its clarity grade, and ultimately it’s value.  Certain flaws can add character to a diamond, such as a diamond with a small garnet crystal trapped in it would be a great conversation piece and a personal touch for someone whose birthstone is a garnet.

Feather

Internal fracture; may be transparent, but usually has a whitish appearance when seen at right angles to the break.  Small feathers do not usually affect a diamond's durability unless they reach the surface on the top of the stone, a location that's prone to accidental blows.

Pinpoint

Small, rounded inclusions; which at 10x magnification is visible, but not large enough to be distinguishable as an included crystal.  These light or dark crystals can appear by themselves or in clusters.  Larger clusters of minute pinpoints can create a hazy area in the diamond, called a cloud, which will affect the diamond's clarity, often incorrectly referred to as carbon spots.

Included Crystal

A transparent or dark crystal of a diamond or mineral trapped within the diamond during its growth.  Included crystals aid in the identification for a diamond, and give it character though if visible to the naked eye can detract from its beauty, which will affect the clarity. 

Natural

This blemish is a portion of the original surface, or skin, of a rough diamond that is sometimes left on a polished diamond, usually on the girdle, to indicate that the maximum yield has been obtained.

- Troy Lob, Graduate Gemologist (GIA), Vice President, Liberty Diamonds