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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Scheelite Details

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Scheelite Information

Scheelite is a calcium tungstate mineral with the chemical formula CaWO4. It is an important ore of tungsten, used in the production of tungsten alloys and for making filaments for electric lamps.

Physical Properties of Scheelite

Optical Properties of Scheelite

Occurrences of Scheelite

Scheelite is found in hydrothermal veins, granitic rocks, and in skarns. It is also found in contact metamorphic deposits and in high-temperature greisen deposits. It is commonly associated with minerals such as calcite, fluorite, apatite, wolframite, molybdenite, and topaz.

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Triplite👆

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Location: France

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