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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Simulants and Imitations of corundum

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Simulants and Imitations

Although there are a number of materials which can have a similar appearance to ruby and sapphire (such as certain spinels, garnets, beni- toite, etc.), all are easily separated by reference to the properties, such as RI and SG.

More dangerous are assembled stones, generally consisting of one or more parts of natural corundum, attached to other materials (generally synthetic corundum). They may show natural inclusions and colours because they are part natural. The key to identifying any assembled stone is to locate the separation plane where the stone is joined together. It must be a distinct join, completely unbroken around the

 

entire stone. It is usually on the girdle, but may be on the pavilion or even the crown. The glue layer often shows curved brush strokes and flat gas bubbles. Since the glue’s RI is quite different from corun- dum, it will stand out in high relief when the gem is immersed in di-iodomethane.

Various kinds of assembled star stones are also possible. One of the most deceptive is simply taking a white star sapphire and coating it with red plastic.

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