Lustre is the intensity of light reflection from the surface of pearls. Lustre depends on the thickness, homogeneity and number of layers of nacre. Lustre makes pearls iridescent and beautiful. The better the reflection on the surface of the gem, the thicker the nacre and the higher the quality of pearl lustre. Lustre is caused by the reflection of light from non-overlapping aragonite plates interspersing with the conchiolin films in the coating of the pearl. This structure breaks up light falling on the surface of pearls and results in rainbow-like iridescence. Experts believe that Akoya pearls have the brightest lustre among cultured pearls, since this type of pearls is grown in cold water and therefore has a dense crystal structure. Lustre is classified into the following categories: AAA - very bright, AA - bright, A+ - average, A - low.