Inclusions in precious stones are like Mother Nature's fingerprints. Scarcely any precious stones are entirely free from inclusions. Most gems contain embedded foreign bodies or exhibit disruptions in their crystal structure. The inclusions can be material of the same sort (e.g. a diamond inclusion within a diamond) or of another sort (e.g. zircon in sapphire). The term "inclusion" is also used to describe cracks, hairline fissures, or hollow spaces which may be filled with liquid or gas. Inclusions can detract from the color and appearance of a stone, but many inclusions are not visible to the eye. More recently, inclusions have been recognized as a decorative element because they can cause exquisite plays of light (e.g. cat's eyes and stars). Some inclusions are so typical of a particular mineral that they can be used as proof of the stone's authenticity.

Internally Flawless is a grade of quality for diamonds and describes a stone's degree of clarity. This criterion is very strictly evaluated and classified according to an internationally recognized evaluative system. "Internally flawless" is the highest grade in this system and is abbreviated "IF." Stones of this grade have no inclusions whatsoever. A diamond is only considered to be "internally flawless" if an expert's eye can detect no inclusions whatsoever under tenfold magnification. Such perfect diamonds are rare exceptions.