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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Kashmir Sapphire Identification: Why Velvety Blue Is Not Enough

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Some Kashmir sapphires are famous for a rich blue colour with a soft, velvety appearance. However, other deposits can produce sapphires that look similar. A beautiful visual effect is not enough to establish Kashmir origin.

What the laboratory compares

SSEF’s 2013 article discusses microscopic growth features, inclusions, chemistry and spectroscopy. Fine particles in some growth zones scatter light and contribute to the characteristic appearance. The shape and arrangement of internal features can provide useful evidence.

The article specifically compares Kashmir and Madagascar material. It describes similarities as well as subtle differences and explains why the evidence must be considered together. A single inclusion or one chemical number should not be turned into a universal origin rule.

Origin does not guarantee quality

Every deposit produces a range of material. SSEF stresses that a Kashmir label does not automatically mean an exceptional stone. Colour, transparency, cut and the appearance of the individual gem still deserve careful attention.

Keep descriptions precise

A phrase such as Kashmir-type may describe appearance while creating confusion about origin. Ask whether the laboratory report actually gives a geographic-origin opinion. Also read the treatment conclusion separately. A stone can be visually impressive without a Kashmir origin, and a genuine Kashmir sapphire can vary considerably in quality. This is a lesson from historical research, not a current price guide.

Sources and further reading

SSEF Swiss Gemmological Institute, Facette, issue 20, January 2013, pp. 6-9, “Kashmir Sapphire”. 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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