The turning back of wave from surface, on which they are incident according to the definite law of reflection. A small part of ray 17.2% of incident light striking the surface vertically is reflected from the surface (the 17.2% is upper limit of reflected light of diamond depends on perfect plane and well polished surface where for quartz is 4.6%) back into the air but greater part refracted into the stone, which may be internally reflected from the facets within the gemstone. Generally both part of reflected light together resulting the luster for example adamantine luster by diamond.