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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UV-Vis Spectroscopy in Gemology: What a Gem’s Light Absorption Reveals

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A gemstone’s colour depends partly on which wavelengths of light it absorbs and which remain available to reach our eyes. UV-Vis spectroscopy measures this interaction. It turns a colour observation into a pattern that a gemologist can study and compare.

What is an absorption spectrum?

An absorption spectrum shows how strongly a material absorbs light at different wavelengths. Peaks and bands can provide information about colour-producing elements or other features inside a crystal. Some useful information lies outside the light we can see.

SSEF’s 2008 article explains a portable instrument developed for testing away from the main laboratory. It discusses applications to colour research and supporting evidence for sapphire origin analysis. The historical equipment specifications should not be assumed to describe all instruments used today.

Why orientation can matter

The spectrum must be measured and interpreted under suitable conditions. The observed colour and absorption behaviour of some gems depend on viewing direction. This makes the measurement setup part of the investigation, rather than an unimportant detail.

A spectrum is supporting evidence

A spectrum can help explain colour, but it should be considered alongside mineral identification, microscopy and other appropriate tests. Think of it as one useful page in the stone’s evidence file. A complex graph is valuable when the analyst explains which question it answers and which uncertainties remain.

Sources and further reading

SSEF Swiss Gemmological Institute, Facette, issue 15, January 2008, pp. 3, “SSEF develops a Mobile UV-Vis Spectrometer”. Read the original SSEF PDF.

This independent educational article explains historical SSEF research in original, simple English. It is not an SSEF publication. No source photographs or illustrations are reproduced.

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