A Brief History Lesson
Back in 1919, Master Cutter Marcel Tolkowsky developed the formula for a 57 facet round cut diamond that trapped all the entering light and returned all of it back out through the top of the stone. The cut was christened "The Tolkowsky Brilliant" which was recognised as having "total internal reflection".
The first modern diamond cutting machine patented in 1876
The Tolkowsky brilliant has been the blueprint for almost every round diamond cut in modern times. Over the years it has been tweaked and fine-tuned to meet changing and differing tastes, but the basic pattern remains the same.
We have learnt that with proper cutting any light entering a stone should be reflected back out of it again, (internal reflection) This effect generates the desired fire, brilliance and sparkle (collectively known as the scintillation) for which diamonds are famous. The most ideally cut stones should achieve "total internal reflection". Variances to the strict formula of angles and proportions will cause the stone to suffer "leakage", thus affecting the overall brilliance and look of the stone.
When judging a diamond's make, it's not just proportions and angles that have to be considered. Yes, a well cut diamond
will ensure that all the "mirrors" of the diamond are working in coordination producing maximum scintillation, brilliance
and internal reflection. But saying that, equal in importance is the quality of the cutter's workmanship.
This can be assessed by the facet alignment (where each individual facet meets its neighbour) and in the stone's symmetry. Finally, the cutter must make sure that the stone has a fine polish on it. This is the finishing touch on the stone and ensures that sufficient light can enter the stone to produce all that brilliance. It also is the external shine of the diamond. A well-proportioned diamond with poor polish would be like making a fine mahogany table and leaving it with a sanded, unfinished top.