Culet

The culet is the very bottom of a faceted diamond — either a small facet polished into the bottom of the pavilion or a point where the pavilion mains meet. Modern round brilliants almost always have either a None (the pavilion converges to a true point) or Very Small culet. GIA grades culet size on an eight-step scale: None, Very Small, Small, Medium, Slightly Large, Large, Very Large, Extremely Large. Anything Medium or larger is visible through the table as a dark spot.

The culet exists for two historical reasons. Old European cuts and antique round brilliants from the 1800s and early 1900s carry visible culets — sometimes 5% to 10% of the diameter — because the cutters left a flat facet at the bottom to reduce the chance of the pavilion point chipping during setting. Cutting and setting technology have improved enough that modern rounds can carry a true point without durability risk. The other reason is symmetry: a culet facet is easier to align than a true point, which is why some mass-market commercial rounds still carry Very Small or Small culets.

A Medium or larger culet shows through the table as a visible dark hexagon or octagon. The effect is sometimes mistaken for a clarity inclusion at the center of the stone; the difference is that a culet shows as a sharp-edged geometric shape and stays in the same place as the stone rotates, while a clarity inclusion moves with the facets. GIA reports note the culet under "Finish" in the proportion summary; a culet larger than Small is flagged in the comments.

For lab-grown rounds and commercial naturals, None or Very Small is the modern standard. Old European cuts and Old Mine cuts (the antique cushion-shaped predecessors of the modern brilliant) routinely carry Slightly Large or larger culets — the visible culet is part of the antique character and is not a flaw on a 1920s-vintage stone. A modern brilliant marketed at antique pricing should not carry a visible culet; if it does, the cut is either commercial-grade or recut from an older stone.

The culet does not affect light performance materially at None to Small. A Medium or larger culet reduces brilliance because light can escape through the bottom facet rather than reflecting back through the pavilion mains; AGS Light Performance grading penalises culet size at Medium and above.