The concept of inclusions is used in research in a very wide sense: under these term are grouped all optically detectable irregularities in the interior of a mineral -and thus also of a gemstone- but not purely chemical inhomogeneities, which can only be ascertained by crystal chemistry. Classed equally as inclusions are intrinsic inclusions and twinning, structural fault, and damage which has occured subsequently in the form of fractures and cleavage cracks.