Every diamond contains natural "birthmarks" in the form of minute traces of other minerals arisen during the crystallization process. The type, distribution, shape, colour and size of these intrusions determine the clarity of a diamond. They interfere with the passage of light inside a diamond, thus bringing down its brilliance. The more and larger the intrusions are, the greater the influence would be.

A higher clarity indicates fewer inclusions. Highly trained experts would determine the clarity of a diamond with a 10-power magnifying loupe recognized by GIA, giving it a self-explanatory grade referred to the table below. In general, the fewer defects a diamond contains, the higher its worth would be. Most diamonds graded S1 or above do not reveal inclusions visibly to the naked eye.