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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Alexandrite or Colour-Shifting Chrysoberyl? Why Lighting Matters

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A gemstone can look different under two lamps without necessarily qualifying as alexandrite. Lighting changes the colours available to illuminate a stone. A narrow-spectrum lamp can therefore create a striking visual shift that needs careful interpretation.

What makes alexandrite different?

Alexandrite is a variety of chrysoberyl associated with a definite colour change caused by chromium. SSEF’s 2012 news slides compare this with a chrysoberyl that looked different under a particular LED but showed almost no main colour change between daylight and incandescent light.

Why the comparison conditions matter

A specialised lamp can emphasise only part of a gemstone’s response. The appearance under that lamp is interesting, but it should not replace a comparison under standard lighting conditions. The same caution applies to photographs whose lighting has not been explained.

SSEF’s later public article reinforces the need to consider an appropriate colour change together with chromium-related evidence. A small shift towards brown in ordinary chrysoberyl is not enough by itself.

Questions to ask about a colour-change gem

Ask which two light sources were used, whether the main hue changed and whether the laboratory identified the material as alexandrite. Chemical and absorption evidence help explain the cause of the effect. A seller’s dramatic LED demonstration should be treated as a viewing observation, not a complete identification.

Sources and further reading

Michael S. Krzemnicki, SSEF, News from the SSEF, SGG Zentralkurs, April 2012, Flüeli-Ranft, slides 11–12, “Alexandrite versus Chrysoberyl” and “This is not Alexandrite”. Reviewed from the supplied PDF; a public direct link to this exact slide file was not verified.

Supporting source: M.S. Krzemnicki, To be, or not to be, that is the question: chrysoberyl versus alexandrite, first published in SSEF Facette 27, June 2021, p. 23.

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

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