Windowing:
A term applied to a gemstone when the pavilion facets are not cut at a critical angle to reflect the light back to the observer. This is done when a gemstone isn't cut to its maximum potential. Windowing can effect the value of a gemstone, as well as optical performance and color saturation. An easy way to tell if your gem is causing a windowing effect is by looking straight through it, if you can see through the backside of the gem without reflections of either light or color, then your gem is causing this phenomena.
Some photos below are examples of windowing. These are natural Citrine from my personal collection. Photos were taking with my iPod, images may be a little fuzzy up close.
This photo are the two citrine stones shown under an LED light. First of all, you can see how the stone on the left isn't as saturated in color.
Photo 1:
The two Citrine Gemstones are both classified as a round brilliant because of the triangular and kite shaped facets along the crown (the top). The table facet (the flat table on the top of the gem) is large. Along the pavilion (the underside of the gem) is more shallow which causes the travel of light to go through the back of the gem with very light reflected light; which causes less dispersion of light (brilliance).
A simple comparison of the two gemstones on an aluminum gem tray.
Photo 2:
The same two stones under normal sunlight. Notice the two stones how the one on the left isn't as brilliant as the one on the right? This is the prime example of windowing. As you can see the facets on the gem on the right not only makes the color deeper, but the facets on the pavilion are cut along the critical angles, which causes the light to reflect back to the viewer, which also creates dispersion (brilliance) enhancing the overall optical performance. A gemstone cut this way makes it more valuable then that of a windowing gem.
An "X" is marked on a piece of paper with the windowing gem covering the mark.
An "X" is marked on a piece of paper with the Good Cut gem covering the same mark.
Photos 3 & 4:
Up close, I have marked a blue "X" on a piece of white computer paper. Putting each gemstone over that same mark, these are the optical effects distinguishing the window gem and a Good Cut Gem. The "X" on the windowing gem is solid and legible. While the "X" behind the Good Cut Gem is a little more distorted. A windowing gem is also known as "Commercial Grade."
Extinction is a term used by Gemologists for the absence of color; an area that shows black.
