Ok - let's see if this works!!!!
Typical problems with gemstone cuts:-
Windowing
A window occurs in a gemstone when the cutter has cut the gemstone too shallow (normally). When viewed from above, it appears as a colourless part of the gem that you can actually see through (hence the name “window”). The effect of this is that light passes out of the back of the gemstone rather than being reflected back up to your eye so the gemstone looks less desirable. Typically, if you place the gemstone over a magazine or newspaper you will be able to read the words through the windowed area.
Windows with a view!!!
A window is not necessarily when you tilt the gemstone and look at it slightly at an angle from the top out to the side of the gemstone. Sometimes you'll be able to see what looks like a window going to the side of the gemstone. However this is caused by the positioning of facets. In an ideal cut, all facets will be in the right place so (hopefully) you can't see through any of the gem but in reality most gems aren't precision cut. Most pale coloured gemstones when turned at an angle will have a paler section. Unless you can see through it when you look through the top it's not a window to be worried about.
This is NOT a window. Windows should only be judged from the top of the stone.
Not a good photo but can you see that there are no areas that you can see through? There are no windows in this gemstone.
Bow Tie
A “bow tie” effect typically occurs to pears, ovals and marquise cuts. It looks like a darker area spread through the middle of the gemstone. Certainly in diamonds, it will affect the look and stops the light being reflected properly. Again, this is due to poor cutting.
Extinction
In most gemstones light you see returned to the eye is what’s called “brilliance”. If you rock a gemstone gently back and forth, you may see areas that appear black (not just darker tones but actual black area). This is called “extinction”. Depending on the amount of extinction, depends on whether it would affect the look of the gemstone. I don’t mind a small amount of extinction in some darker coloured gemstones but can’t tolerate it in lighter ones.
The GIA define extinction as “areas of darkness visible when the stone is judged for brilliance. It is caused by blackness in the stone and the proportion angles”.
I like the following definition as I think it explains it better and is from The American Gem Market Network Grading Manual:
When grading extinction, the stone is held face up over a plain white background under diffused daylight lighting, and a judgment is made as to how much (what percentage) of the stone reflects back the true color of the gem and does not "black out." The visible surface of the stone represents 100%. When the stone is tilted slightly in any direction, any extinction present will appear to move from facet to facet. The percentage of the stone which is "extinct" usually remains constant, so an estimation can be made as to what percent of the total area is extinctive. Care should be taken not to confuse dark tone with extinction. Extinction is black, with none of the hue of the stone apparent.
This is a Ruby that shows both brilliance and extinction:
Four blue sapphires showing a variation in saturation and tone. Stone 1 possesses a light tone and low saturation. Stone 2 is close to ideal in both tone and saturation. Stone 3 has greater saturation than Stone 2 in some areas, but its overall tone is too dark and it shows too much extinction. Stone 4 is so dark in tone that its saturation is reduced. Note that inclusions are far more visible in stones of light tone than those of dark tones.
Here's one of my gemstones. It's an Umbalite Garnet and as you can see it has areas of extinction. However, the overall gemstone is still bright and lively and so it doesn't detract too much.
Hope this helps!
