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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Colored Gems Weight Estimation Tables

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 Weight Estimation Tables for Colored Gems

 

To obtain approximate weights of the following shapes of colored stones multiply by the following measurements to millimeters. A listing of the specific gravities is below.

 

Round Faceted = ...................Diameter x diameter x depth x SG x .0018

Oval Faceted = .......................Diameter x diameter x depth x SG x .0020

Emerald Faceted = .................Length x width x depth x SG x .0025

Rectangle Faceted = .............Length x width x depth x SG x .0026

Square Faceted = ...................Length x width x depth x SG x .0025

Marquise Faceted = ...............Length x width x depth x SG x .0017

Pear Faceted = ........................Length x width x depth x SG x .0018

Heart Faceted = ......................Length x width x depth x SG x .0018

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