Some treated yellow and blue-to-green diamonds show a peculiar color zoning when viewed through the pavilion. The culet or keel-line area on such stones exhibits a bright yellow or blue color that contrasts with the otherwise lighter (and/or different) body color of the diamond (which does not show any other strong color zoning). Such peculiar color zoning near the culet or keel-line is a strong indication that the stone has been treated by electron irradiation. Although rare, this feature can be useful in identifying irradiated diamonds, especially some green diamonds, for which few criteria are available to separate natural-color from laboratory-treated stones.
