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Diamond Light Performance

April 27, 2022

Did you realise that these images used for years about light performance are wrong!

Once upon a time, a graphic designer was instructed to draw what happens in a shallow, ideal, and deep-cut diamond. See below … It was incorrect!. The middle ‘best diamond’ is actually the worst of all!

For at least four and a half decades this mistake has been perpetuated on thousands of websites. Why don’t educated people challenge clearly wrong things?

The critical angle is Gemmology 101. A diamond that is shallow must have a pavilion angle of less than 24.5 degrees for light to leak out. No round diamond was ever cut with a 35 degree or less pavilion angle. Only rose cuts.
The centre diamond has a pavilion angle of 45 degrees. No bending of entry rays, because there is no refraction, only two internal reflections. The light enters and leaves parallel, so you see your own head and no lights and can make the centre of the diamond sparkle.

In the industry, we call this diamond a “Nail Head” and it is the deadest dullest of all diamonds. More than 95% of all round diamonds have pavilion angles between 40 and 42 degrees.
So guess what – I hope you already did – the deep diamond never ever existed! It requires a pavilion angle greater than 53.5 degrees. Cutters turn that rough diamond on its side and cut and polish a pear-shaped diamond.

If you want to see what really happens check out the reference chart on Ideal-Scope:

BTW they are not all correct either – Does anyone want to say which ones are wrong?

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