ex·tinc·tion | ik-ˈstiŋ(k)-shən
Dark or black spots in colored stone
There are several reasons why a cut stone will display dark spots. Three of them include absorption, windowing, and object reflection. The fourth is extinction. This refers to when light enters a gem and bounces off of a pavilion facet but does not return as observable by the viewer of the gem. This creates strong contrast with the adjacent facets that lead to incredibly dark and sometimes nearly black spots on the stone. The severity of extinction is directly related to the steepness of the pavilion on the gem. This can be used in lighter gems to increase the saturation of the color of the gem.