Inclusions in gemstones are like Mother Nature's fingerprints. Almost no gemstones are completely free of inclusions. Most gems contain embedded foreign bodies or have breaks in their crystalline structure. Inclusions can be materials of the same type (e.g. a diamond that includes inside a diamond) or other materials (e.g. sapphire zirconines). The term "inclusion" is also used to describe cracks, hairline cracks or gaps that can 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 play 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 authenticity of the stone.

Internally Flawless is a grade of quality for diamonds and describes the degree of clarity of a stone. This criterion is very rigorously evaluated and classified according to an internationally recognized evaluation system. "Internally impeccable" is the highest grade in this system and is abbreviated "IF". Stones of this grade have no inclusions whatsoever. A diamond is considered "internally impeccable" only if an expert's eye cannot detect any inclusion under tenfold magnification. Such perfect diamonds are rare exceptions.