A facet on the side of a native-cut, partly-coloured gemstone (usually a sapphire) that is cut so that the strongest colour is at the base and the stone consequently appears to have a uniform colour when viewed through the table but appears clear when viewed from the side. A facet cut on a diamond with a rough or coated surface to permit a visual examination of the interior. The process is called 'opening the diamond'. A facet on some square-cut emeralds or other so cut gemstone that permits light to escape and through which an object can be seen when viewed from the table.
From: An Illustrated Dictionary of Jewelry, autor: Harold Newman, publishers: Thames and Hudson