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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What is the definition of hardness? What is the Hardness Scale? How is it used in mineral testing?

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Hardness Scale (Friedrich Mohs Scale)


The bonds holding atoms together determine a mineral's

hardness.

In 1812, the Austrian mineralogist Friedrich Mohs devised a

scale of hardness that is still in use today. He selected ten

minerals as standards and arranged them so that any mineral on

 the scale 


would scratch only those below it. Everyday objects can be used 

to test where a mineral fits into the scale. A fingernail has a

hardness of 2.5, and a penknife is 5.5. Minerals of six and above 

will scratch glass; glass will scratch apatite and other minerals 

below it.

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