Round Brilliant Cut
Also called round cut, brilliant cut, rbc
Round brilliant cut is the standard circular diamond cut, made up of 58 facets including the culet, or 57 when the point is left closed. Its proportions are worked out to send as much light as possible back up through the table to the viewer.
In other words, the classic round diamond shape, cut with a fixed facet pattern that is tuned to throw back the most light.
How the facet pattern works
The crown above the girdle carries the table, eight bezel facets, eight star facets, and sixteen upper girdle facets. Below the girdle the pavilion has eight main facets and sixteen lower girdle facets, meeting at a point or a small culet.
The angles matter more than the count. When the pavilion angle is right, light entering the crown bounces off two pavilion facets and exits back through the top. When it is too shallow or too deep, light leaks out of the bottom and the stone looks glassy.
Why it matters for rendering
Round brilliants are the shape buyers compare hardest, because they have seen thousands of them. A render with a lazy facet pattern gets spotted straight away, even by people who could not say what is wrong.
We model the full facet structure and let the renderer trace light through it, so the pattern of bright and dark facets shifts as the piece turns. That is why the turntable videos carry more weight than a still: the stone behaves.
Questions about Round Brilliant Cut
How many facets does a round brilliant have?
Fifty eight, counting the culet at the bottom point. Stones cut with a closed point are counted as fifty seven.
Why is the round brilliant the most popular cut?
It returns the most light of the common shapes, and its proportions are standardised enough that buyers can compare stones between sellers with some confidence.