| Ideal Diamond proportions
To produce maximum brilliancy, light-dispersion and scintillation, it is necessary to compute mathematically the best angles and proportions to cut a diamond. Marcel Tolkowsky published such a computation in 1919 in a book entitled Diamond Design. This set of angles and proportions came to be called the "ideal" cut.
In an Ideal Cut every facet has to be placed at an exact angle and the proportions are so calculated as to create an "ideal" balance between maximum brilliance (the return of the light to the eye) and dispersion or "fire" (the prism effect that separates white light into its spectral colors).
The figure below shows the principal elements of the "ideal" cut as calculated by Tolkowsky.
Tolkowky determined that the pavilion depth should equal 43.1 percent of the girdle diameter.
Today most cutters and experts agree that even a deviation as small as two degree from Tolkoskys theoretical pavilion angle results in a noticeable darkening of the stone and an obvious loss of brilliance. On the other hand a decrease of two degrees in the pavilion angle will produce a fish-eye effect; this is a grayish girdle reflection seen through the table of the stone. The additional light leakage from the pavilion also gives the stone a very glassy appearance.
What is known as an ideal cut nowadays is a little more elastic than the proportions originally proposed by Marcel Tolkowsky: a 53 percent table; 16.2 percent crown height; and 43.1 percent pavilion depth.
In 1972 the American Gem Society when it ratified its 0-to-10-cut-grade adopted the following standards:
Table diameter: 53 to 57.5%
Perfect alignment of crown and pavilion facets.
Mirror-like finish with no polishing lines or wheel burns.