Gemstone Windows and Dark Patches: What to Look for in a Cut Gem

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AI-generated educational illustration. Conceptual only; not a photograph of an actual specimen or location. 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 fac...
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Can You Name These Gemstone Optical Effects? A 60-Second Challenge

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Gemstones do more than sparkle. Some perform optical tricks so dramatic that one tilt can change the entire stone.

The challenge

Blue flash sliding over a gray stone: Labradorite — labradorescence.

A moving band of light on a cabochon: Cat’s-eye chrysoberyl — chatoyancy.

A star appearing under one light: Star ruby or sapphire — asterism.

Different colors from different directions: Tanzanite or iolite — pleochroism.

Green in daylight, reddish under warm light: Alexandrite — color change.

Rainbow colors in a pale cabochon: Opal — play-of-color.

Gemology rule: An optical effect can support identification, but it is not a complete identification by itself. Lighting, cut, and treatment can all change what you see.

How many did you know before reading the answers? Send this to the friend who always says every shiny stone is a diamond.

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