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Round Cut Diamond Info

A Round Brilliant Cut Diamond is a Diamond or other Gemstone, cut in a particular form with numerous Facets so as to have exceptional brilliance. The shape resembles that of a cone and provides maximized light return through the top of the diamond.
Even with modern techniques, the cutting and polishing of a diamond crystal always results in a dramatic loss of weight; rarely it is less than 50%. The Round Brilliant Cut is preferred when the crystal is an octahedron, as often two stones may be cut from one such crystal. Oddly shaped crystals such as Macles are more likely to be cut in a Fancy Cut.
The Original Round Brilliant Cut was developed by Marcel Tolkowsky in 1919. The modern Round Brilliant consists of 58 Facets (or 57 is the Culet is excluded). Ordinarily today, Round Brilliants are cut in two pyramids placed base to base: 33 on the Crown (the top half above the middle/Girdle of the stone), truncated comparatively near its base by the table, and 25 on the Pavilion (the lower half below the girdle), which has only the apex cut off to form the Culet, around which 8 extra Facets are sometimes added. In recent decades, most Girdles are Faceted. Many Girdles have 32, 64, 80, or 96 Facets; these Facets are not counted in the total. While the Facet count is standard, the actual proportions (Crown height and angle, Pavilion depth, etc.) are not universally agreed upon. One may speak of the "American Cut" or the "Scandinavian Standard" to give a couple examples.
The relationship between the crown angle and the Pavilion Angle has the greatest effect on the look of the diamond. A slightly steep Pavilion Angle can be complemented by a shallower crown angle, and vice versa. Other proportions also affect the look of the Diamond:
  • The Table ratio is highly significant.
  • The length of the Lower Girdle Facets affects whether Hearts & Arrows can be seen in the stone, under certain viewers.
  • Most Round Brilliant Diamond have roughly the same Girdle thickness at all 16 "thick parts".

          Facet Proportions & Names




Figures 1 & 2 show the Facets of a Round Brilliant Cut Diamond







Figure 1 assumes that the "thick part of the Girdle" is the same thickness at all 16 "thick parts". It does not consider the effect of indexed upper Girdle Facets.










Figure 2 is adapted from Figure 37 of Marcel Tolkowsky's Diamond Design, which was published in 1919. Since 1919, the lower Girdle Facets have become longer. As a result, the Pavilion Main Facets have become narrower. 

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