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Originally in 1945, the divisions were based on the ring inside diameter in steps of 1 ⁄ 64 inch (0.40 mm). [6] However, in 1987 BSI updated the standard to the metric system so that one alphabetical size division equals 1.25 mm of circumferential length. For a baseline, ring size C has a circumference of 40 mm. [7]
Original file (775 × 625 pixels, file size: 6 KB, MIME type: application/pdf) This is a file from the Wikimedia Commons . Information from its description page there is shown below.
Typically a round brilliant 1.0-carat (200 mg) diamond should have a diameter of about 6.5 mm (0.26 in). Mathematically, the diameter in millimeters of a round brilliant should approximately equal to 6.5 times the cube root of carat weight, or 11.1 times the cube root of gram weight, or 1.4 times the cube root of point weight.
Diamond proportions and facets, for the round brilliant cut. The modern round brilliant (Figure 1 and 2) consists of 58 facets (or 57 if the culet is excluded); 33 on the crown (the top half above the middle or girdle of the stone) and 25 on the pavilion (the lower half below the girdle). The girdle may be frosted, polished smooth, or faceted.
The 813-karat diamond (which is roughly 407 times the size of the average US engagement ring, for reference) was mined from Lucara's "Karowe" mine in Botswana in late November of last year.
Diamond proportions and facets, for the round brilliant cut. The modern round brilliant-cut was developed by Marcel Tolkowsky in 1919. The ideal proportions are 100% diameter, 53% table, 43.1% pavilion and 16.2% crown. The girdle and culet (if any—not part of Tolkowsky's original design) are cut from the ideal brilliant.