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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Paraiba Tourmaline Origin: Why Neon Blue Cannot Prove Brazil

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An intense blue or green-blue tourmaline can be beautiful without revealing the country where it formed. Copper-bearing tourmalines from Brazil and parts of Africa can have similar appearances. Colour is therefore an unreliable shortcut for geographic origin.

What trace elements can reveal

SSEF’s 2007 article compares material from Brazil, Nigeria and Mozambique. It discusses trace-element patterns, including lead, gallium and bismuth, studied with methods such as LA-ICP-MS. These are small chemical clues rather than features visible to the naked eye.

The article notes substantial overlap in chemical composition. Useful distinctions emerge from careful comparisons and data interpretation, rather than a rule that one element always identifies one country.

Why reference samples matter

A laboratory needs reliable samples from known sources to interpret the chemistry of an unknown stone. A measurement becomes much more useful when it can be compared with a documented reference collection. That comparison still has to account for natural variation.

Keep variety and origin separate

A gemstone’s variety name, its treatment history and its geographic origin are separate report questions. Ask whether an origin statement is actually included and supported. A bright colour description, a sales photograph or the word Paraiba should not be treated as independent proof of Brazilian origin. This post explains the historical research rather than setting today’s naming rules.

Sources and further reading

SSEF Swiss Gemmological Institute, Facette, issue 14, January 2007, pp. 8, ““Paraiba” Tourmalines from Brazil and Africa”. 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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