I found this definition of "window" on the ACStones website:
"Window: A window is an area under the table of a stone that does not reflect light. It's called a window because light goes through it like window glass. It is almost always undesirable and occurs when pavilion facets are cut below the critical angle for a gem material. To test for a window, hold a gem, face up, with the table parallel to a sheet of newspaper and look through it. If you can read the print, the stone has a window. Most native cut stones have large windows as using low pavilion angles allows a larger stone from a given piece of rough."
Pink jewel - it's also important to note that a window is only one that you can see when you look at a gemstone head on. It's not when you look at a window and see a similar effect when the gem is viewed from the side - this is called a tilt window. The photos below show a tilt window and this is absolutely nothing to worry about.
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