IDEAL-CUT DIAMOND : Why the ‘American-Cut Diamond’ Endured

When accurately executed, the best proportions of the round brilliant cut result in the most beautiful white diamond, exhibiting the greatest refulgence. This ideal diamond now goes by the name ‘ideal-cut diamond,’ also known as the ‘American ideal-cut,’ ‘American-cut diamond,’ and ‘Tolkowsky brilliant.’

The Tolkowsky Brilliant

The name ‘Tolkowsky brilliant’ came from the 21-year-old engineer who unveiled the ideal proportions of the brilliant-cut diamond. Marcel Tolkowsky of Belgium reduced to a mathematical formula the proportions of an excellent brilliant, which gives the most colorful flashes with the least sacrifice of white light. Such ideal diamond sparkles with white light and shines with the colors of the rainbow. When moved, the Tolkowsky diamond also produces rapidly changing flashes, a quality known as ‘scintillation.’

The proportions of the ideal diamond that Marcel Tolkowsky unveiled offered a reliable model from which to design the perfect gem. By observing this optimum set of ratios among the different parts of the brilliant, cutters found it easier to shape a rough crystal into the perfect gem.

Table of the Ideal-Cut Diamond

The table was the first facet that cutters made on a diamond. By the 1860s, the width of the table was 5/9 of the girdle’s diameter. This rather closely approximates to the calculation of Tolkowsky, who puts the width of an ideal diamond’s table at 53% of the breadth of the gem.

Angles of the Ideal-Cut Diamond

Tolkowsky’s calculations in 1919 specifically unveiled at what angle the bezel and the pavilion of an ideal brilliant should meet. Measured from the girdle, these optimum angles were significantly different from each other. Tolkowsky recommends that the pavilion be inclined at only 40.75°, and the bezel less steep at 34.5°.

With narrower angles between the bezel and the pavilion, the brilliant grew thinner. While the brilliants from the 1820s were about as thick as they were wide, the ideal-cut diamonds, conforming to Tolkowsky’s calculations, had a height of only 6/10 of the width, or 59.3% of the girdle’s diameter to be exact.

Proportions of the Ideal-Cut Diamond

The height of the crown and of the pavilion among diamonds varied in relation to the depth of the entire gem. Since as early as the 1820s, the crown was only 1/3 of the total height of the brilliant, thus leaving the depth of the pavilion at 2/3. Tolkowsky, on the other hand, put the thickness of the ideal diamond’s crown at 27.3% or a little more than 1/4, and therefore the pavilion at 72.7% or less than 3/4 of the height of the whole stone.

Indeed, the crown of the brilliant used to be much thicker. The difficulty in cutting, as well as the greater loss of weight otherwise necessary, compelled lapidaries to leave the brilliant thicker above the girdle. Before the practice of sawing diamonds, cutters had to grind away about a fourth of the rough stone to produce the broad plane surface of the crown, and this at a direction against the grain of the crystal. Hence, carving the table required the most work and the most waste in material. In consequence, cutters left the crown thicker, which was too thick for Tolkowsky and today’s standard of the ideal diamond.

The use of saw upon diamonds led to much thinner and therefore much finer brilliants. Unlike grinding, sawing corners or edges off diamonds resulted in whole fragments, which may in turn produce separate gems, thus enormously reducing the loss in raw material. Hence, artists discarded their hesitation to slice large chunks off their diamonds, and resolutely cut the crown of the brilliant much thinner. As a result, the proportions of the brilliant drew closer and closer to the dimensions of Tolkowsky’s brilliant cut.

Facets of the Ideal-Cut Diamond

To unfold most of the stones’s brilliance, even the facets upon the crown of the ideal-cut diamond varied in the steepness of their inclination. For instance, while the bezel ideally sloped at an angle of 34.5°, the star facets slanted at an even flatter angle of 15° as they touch the table.

As a result of the introduction of the steeper facets, both on the crown and on the pavilion, the greater part of light entering the brilliant travels toward the crown, instead of leaking through the back, where the brightness would be unseen and therefore unappreciated.

The Tolkowsky Brilliant Today

To this day, the proportions of Tolkowsky’s ideal brilliant hold. Other cutters, equipped with far more advanced technology, took his proportions much further and refined his specifications, but the idea is essentially the same.

Indeed, today the round brilliant remains the most important cut, not just for diamond, but for other transparent stones as well. Many variations have come out in recent years, yet these typically retained the shape and proportions of the ideal-cut diamond.

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About the Author

Jomar Pandan

Jomar ‘Deomar’ Pandan is the author of Power Birthstone. He’s on the fourth draft of a high-fantasy novel, Cybelym, and the first of a historical fiction, Gem Faerie. He hopes to release a nonfiction book about diamond soon. Follow him for updates. Or see his jewelry and designs.