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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Cause of Color in Gemstones

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Cause of Color in Gemstones

Selective Absorption

Selective absorption is one of the main causes of color in gemstones. Certain gemstones have atoms or ions in their crystal structure that selectively absorb certain wavelengths of light. The absorbed wavelengths correspond to the colors that are seen by our eyes, giving the gemstones their characteristic color.

Residual Color

Residual color refers to the color that remains in a gemstone even after all impurities have been removed. This residual color is caused by the presence of trace elements or defects in the crystal lattice of the gemstone.

The Transition Elements

The transition elements, such as chromium, iron, and titanium, can cause color in gemstones. These elements can replace certain atoms in the crystal structure of the gemstone and give rise to different colors.

Crystal Field Transitions

Crystal field transitions occur when electrons in the crystal lattice of a gemstone absorb or emit light energy. These transitions are responsible for the colors observed in certain gemstones.

Charge Transfer Process

The charge transfer process involves the transfer of electrons between different elements or ions in a gemstone. This process can result in the creation of color centers and contribute to the overall color of the gemstone.

Color Center

A color center is a defect in the crystal lattice of a gemstone that can absorb or emit specific wavelengths of light, resulting in color. Color centers can be caused by various factors, such as radiation or impurities.

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