The beauty of a polished diamond is a factor of its ability to absorb, refract, reflect and to transmit natural light, causing it to shine and sparkle with intensity. The success of a specific cut is an accumulation of various visual effects: brightness, which is the amount of light that is reflected from the stone; fire, which is the effect caused of the white light being split into the colors of the rainbow; and scintillation, which is the degree of sparkle that is so typical of the diamond, and is caused by light being reflected off facets from within the stone.
The diamond shape that is popularly considered to best achieve all three effects is the round cut. It has been around for centuries, but in its modern form will be celebrating its centenary year in 2019. Called the “Ideal Brilliant Cut,” its father was Marcel Tolkowsky, a member of a renowned family of diamantaires that had moved from Byalistok, Poland, to Antwerp, Belgium, in 1840s.
In 1919, at the age of 21, Marcel was a studying for a PhD in mathematics at the University of London. His doctoral thesis was entitled “Diamond Design: A Study of the Reflection and Refraction of Light in Diamond,” and it changed the diamond industry forever. In it, he proposed a set of optimal proportions for a 58-facet round cut (57 if there is no culet), which he suggested would minimize the amount of light escaping out the sides or bottom of the diamond, and maximize the light emerging through the top, or table, from where it originally had entered.
Tolkowsky proposed that a round brilliant cut diamond should be cut to the following specifications:
The Crown Angle, which is the diagonal from the edge of the table facet to the girdle, should be 34.5°.
The Pavilion Angle, which is the diagonal from the edge of the girdle to the culet, should be of 40.75°.
The Table Diameter, which is the diameter of the table facet divided by the total diameter of the stone, should be 53%.
The Total Depth of the diamond, which is the depth of the stone from table to culet, divided by the width of the diamond, should be 59.3%.
The Crown Depth, which is the depth of the upper section of stone (crown height), from table to girdle, divided by width, should be 16.2%.
The Pavilion Depth, which is the depth of lower section of stone (pavilion depth), from girdle to culet, divided by width, should be 43.1%.
Most diamond cutters will claim that Tolkowsky’s optimal proportions are in fact impossible to meet, because it calls for a 0% knife edge girdle, which is where the crown meets the pavilion. The quality of the round cut is essentially the degree to which the diamond cutter manages to meet Tolkowsky’s ideal standards.
Be that as it may, more than 75 percent of the polished diamonds produced in the world today are intended to approximate Tolkowsky’s Ideal Brilliant Cut. As the Gemological Institute of America noted: “Virtually all round diamonds are brilliant-cut and for good reason. When cut as a round brilliant, the diamond’s angle helps to enhance its fire and brilliance.”
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