What is an American Cut Brilliant

Also known as an Ideal Cut

A diamond proportioned in accordance with the calculations published in 1919 by mathematician Marcel Tolkowlsky. 
"The only aim being to obtain the liveliest fire and the greatest brilliancy."

Girdle diameter considered as 100%: table diameter, 53%; crown thickness, 16.2%; pavilion thickness, 43.1%; table-to-culet depth, 59.3% plus girdle thickness; angle of crown facets, 34.5 degrees; angle of pavilion facets, 0.75 degrees American Gem Society grades of cutting are defined in terms of departure from such proportions and finish.

The Proportions of an Ideal cut according to GIA