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In the first article, I explored common areas of agreement in the cutting of the Ideal Round Brilliant. The target center of the Ideal sweet spot for all groups was discovered for three of the seven parameter dimensions that define the round brilliant cut. The most critical two of the three, the pavilion main angle and the crown main angle, are close to those of early innovators in the evolution of the Ideal Cut, especially the American diamond cutter, Henry Morse, and the Belgian mathematician, Marcel Tolkowsky. This concluding article answers the question: "What about the center of the sweet spot for the remaining four of the seven parameters defining the round brilliant cut?"
There is general agreement that the interrelationship of the individual proportions determines a diamond's performance and beauty. We can explore the ranges of the other parameters in the context of the sweet spot center of best table sizes, pavilion angles and crown angles. High in importance along with pavilion angle and crown angle is the length of the pavilion halves, (lower girdle facets).
In the early 19th century and prior eras, most of the area of the pavilion was occupied by the main facets, which dominated the diamond's reflection pattern. As can be observed today in "Old Mine" Cut diamonds from that era, the halves were small compared to those of the modern round brilliant. At that time, the pavilion halves extended less than halfway to the culet. In contrast, Tolkowsky indicated in his book 'Diamond Design' in 1919 …