Marcel Tolkowsky invented the Ideal Cut Proportions for the modern Round Brilliant Cut.

From the Tolkowsky website:

“It is the cut that is primarily responsible for the fire, brilliance and scintillation of a diamond and it is the only factor that is influenced by man.

Tolkowsky Ideal Cut diamonds unlock the perfect proportions needed to maximize brilliance and dispersion of light. Invented by Marcel Tolkowsky in 1919, the Ideal Cut is still the benchmark by which all diamonds are judged to this day.”