How can I tell if a gem is "windowed"?

In a well cut faceted gem, the pavilion facets (those on the lower half of the stone) should reflect light back out the top or table of the stone. If the facets are cut below the critical angle for the particular material, light will pass right through the stone instead of being reflected back towards your eye. When this happens the gem will lack sparkle and brilliance. Try holding your gemstone over a piece of paper with some text on it. Look down through the table of the gem. If you can read the text clearly through the gem, it has a window.

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