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How to evaluate the brightness of a cut stone

I have asked several gem cutters about how to evaluating the brightness of a cut stone. But no one gives me a satisfied response. I wish someone here can answer my confusion. Thanks in advance.

Hi a simple question but not a simple answer, in your question you say Brightness this could also be interpreted as dispersion of a gemstone. this depends on many varients. (a) Refractive index of a gemstone, high refractive index usually means a gem will have high dispersion, such as Diamond, Zircon, Sphene, Demantoid Garnet etc. the lower the Refractive Index the less brightness/dispersion such as Tourmaline, Beryl thats why most cuts for these gems are different to high dispersion gems. (b) then you have the colour of a gemstone, darker the colour this effects the brightness of the gemstone, such as Sapphire, nice light colours will show reasonable dispersion, but if you have a dark blue sapphire you will get very little dispersion. (c) another major factor is if the gemstone is cut well, if the pavilion facets are well cut and not under the critical angle of the gemstone for facets then the light will bounce back and give brightness. if the facets are below the critical angle or very shallow a lot of the light will travel directly through the gem and not bounce back which will give a dull appearance to the gem than what would be expected. There are many variables but these are the main ones of the top of my head, I am sure some of the other Pro Community members can add to this thanks.

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By brightness I presume you mean the sparkle or lively play of light from within a cut stone. I also presume that you want some scientific measure that will make it easier to judge if one cut stone is “brighter” than another. Unfortunately there is at present no such index and if there were there would still be subjective opinions on its overall utility. Perhaps the only valid test is one where you place two jewels of the same mineral class and also identical cut side by side under the same light and select the best.
If they are from different minerals and also have different cuts then you may find it easier to tell which is the brightest. However, that will be the result from a variety of differing scientific factors such as refractive index etc and not a difficult - to - quantify “brightness” value.

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Hello, in diamonds you have triple excellent, so y know this is the best. For color stones it is an other question: color is most important and also finish: perfect facets, the polishing (with diamond powder) take time, the cut of colored stones must give first the maximum of color depending his color zoning. You can make the stone lighter or darker by cutting. Gems are mostly nice cut, but in commercial goods the cut is low so less brightness (again takes time).
It is not possible to measure it by electronic divises, same for the color of diamonds, there are different tools but there give all different results.

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I had a woman come to me wanting a ring made recently. She only had 2 requirements. It had to be pink and really sparkly. She had no idea what type of stone she wanted. It could be anything in the world but it had to be pink! Sparkly? I sent her some ideas and she just could not get the full picture from a computer screen. I pulled everything I have in pink and set them in a portable display case so she could walk around my shop, home and outside to see what stone she decided had the most sparkle. She and I disagreed on the most sparkly stone I had, so, I find that the sparkle factor comes in the eye of the beholder and not from some gauge, chart of from a machine. Your client has the last word as far as I’m concerned. BTW, money was not an issue with her and she turned down a stunning pink diamond.
All the best,