Gemstone Windows and Dark Patches: What to Look for in a Cut Gem
A coloured gem may have a pale, see-through centre or areas that look almost black. Both affect its appearance, but they do not have the same cause.
What is a window?
A window is an area where light passes through the gem instead of being returned towards the viewer. You may see the background through it, especially near the centre.
Cutting angles and the material's refractive index—how it bends light—help determine whether this happens. There is no single ideal depth for every gemstone.
Why dark patches need a closer look
Darkness can have several causes. The material may absorb a lot of light. Some facets may reflect dark surroundings or the observer. Extinction is another cause, involving light that is not returned to the viewer after internal reflection.
For that reason, calling every black patch “extinction” can be misleading.
A simple viewing routine
Look at the gem face-up in broad, even light. Gently move it and notice which areas brighten, stay dark, or reveal the background. Compare more than one lighting condition.
A window that appears only when the stone is tilted is different from a large window seen straight down. Observe both, rather than judging from one photograph.
These observations help you describe the cut. They do not identify the gemstone, establish treatment, or determine its value on their own.
Source: GIA: Colored Stone Darkness and Brightness.
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