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Dispersion


#1

Dear all
How is possible to estimate and evaluate Dispersion in gemstones?
Thanking you for your comments


#2

You could get a very rough approximation of dispersion just by using Visual Optics and looking at the width of the rainbows in a faceted stones. You could perhaps do a little better by observing the rainbows in your refractometer, if your gem is below the 1.81 limits. The traditional way of measuring dispersion is through the use of a table spectrometer, but these are not cheap and then one must learn the skill of using one. Using narrow bandpass filters would give you a measurement with the refractometer, but the lines may be difficult to see. To me it would make the most sense to estimate very roughly the dispersion of the stone through looking at the rainbows for an aid to identification. If you want a good estimate of the dispersion, just proceed by normal methods to ID the stone and then you can look up the dispersion in a table. Dispersion within a species doesn’t vary that much and the only use for knowing it I can think of is to take advantage of it when faceting certain stones. However, many colored stones don’t have that much dispersion anyway.

It’s become the fashion in cutting in some quarters to increase face up brilliance by using lowered crown angles, but this tends to reduce both tilt brilliance and dispersive fire. Faceters might do better to investigate using higher crown angles to increase both tilt brilliance and dispersive fire. Probably more than you wanted to know…royjohn


#3

Thanking you for all your helpful comments, royjohn