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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The Sapphires of Montana

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When the first prospectors set out for the American west, they were looking for gold, and in Montana, they found their share. But some of these unwittingly lucky miners also found another dense precious mineral — sapphires. Most of the sapphires found in the 1800s in Montana were for industrial use. But in the late 1900s, heat treatment processes were developed that made Montana sapphires one of the most important American-mined gems. Join GIA researchers Nathan Renfro and Aaron Palke as they unearth the history and gemology of these stunning jewels.



Location: Montana, USA

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