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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Emerald Vs Green Beryl

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Emerald can own its green colouration to Chromium and iron, but the question was raised when vanadium coloured green beryl was found in Brazil... Up to this point, it was common that 'emerald is the green variety of beryl, it is coloured by chromium". Most emeralds are coloured in fact by iron and chromium, some other stones can also be colored by vanadium. 

For most gemological laboratories the standard is that if a green stone from the beryl family shows some chromium spectrum and present a chromium percentage higher than 0,1% then it can be called emerald. 
If not it is green beryl... 

This 0,1% "standard" is coming from a Kurt Nassau proposal in order to solve the problem opposing Colombian emerald dealers and brazil or African dealers: As most of the Colombian emeralds which are the traditional emeralds owns their colour to chromium and iron. The new emeralds from brazil were vanadium rich. The Colombian dealer lobby was the stronger, this proposal has been globally accepted. 

You can so have a very saturated green beryl coloured by vanadium but without the 0,1% of chromium and a weak green emerald that can be called emerald despite its weak green color because its host 0,1% of chromium... 

But anyway on the internet an emerald sells better than green beryl... so many people try to be ignorant in order to make better sales and margins. In gem business, it is sad to say but the greed of some is very well completed with the greed and the greater ignorance of the others!


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