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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Powellite details

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Powellite

Powellite is a calcium molybdate mineral with the formula CaMoO4. It was first discovered in 1914 and named after Arthur Powlett, 5th Duke of Cleveland, who had a keen interest in mineralogy.

Powellite has the following physical and chemical properties:

  • Color: Yellow, yellow-green, grayish-green, grayish-white
  • Crystal System: Tetragonal
  • Crystal Habits: Crystals are square prisms, sometimes with a long columnar axis and a short square prism; also as granular and scaly masses
  • Cleavage: Perfect on {001}
  • Fracture: Conchoidal to uneven
  • Hardness: 3.5-4.5 on Mohs scale
  • Luster: Adamantine, greasy, waxy, dull
  • Streak: White
  • Specific Gravity: 4.3-4.5
  • Transparency: Transparent to translucent
  • Optical Properties: Uniaxial (-), may be anomalously biaxial due to strain

Powellite can be found in various localities worldwide, including the United States, Canada, Mexico, Italy, Germany, and China. For more information about Powellite


Powellite Mineral

Location: China

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