A window is a lack of color in the center of a cut stone, so a concentration of color in the periphery. This is seen on a cut gemstone whose the height of the cullet is not deep enough. The cullet of a gemstone must have a height depending of the refractive index of the stone.
So, for example, to do not have a window, a topaz or a peridot must have a higher cullet than an amethyst or a citrine.
