The culet, the bottom point facet of the diamond, is for protective purposes only and serves no added value to the stone. When too large, it can severely take away from the beauty of the stone and can affect the value at least slightly, depending on its size and visibility.
A favorable culet size is small to medium. A pointed culet (or “none” on some lab reports) is not considered a negative, but again, it should not be handled by steel tipped tweezers. On pointed culets, you’ll often see very small chips due to mishandling.
The abrasions that are sometimes noted traveling down the pavilion main facet edges, are more than likely caused in the cutting process. Even though this is often called a culet feature, it is not a culet characteristic since it is above the point area. These minor blemishes can usually be polished away with almost undetectable weight loss.
To check the culet size, look through the table with your 10x loupe. With a Very Small culet, tilting the stone might be necessary to make sure there is a culet at all. A Medium culet will be easy to spot, and you’ll probably be able to distinguish the octagon shape. Slightly Large will be almost visible to the naked eye and you’ll easily distinguish the octagon shape under 10x. A Large to Extremely Large culet will be visible to the naked eye.
When diamond cutters first started fashioning old style cuts, such as the mine and European, they didn’t realize that the large to extremely large culets that they polished on the diamonds, provided easy escape for the light traveling through the stone. A good part of the light entering the diamond took a quick exit through the culet.
The important aspect to keep in mind is how culet size affects brilliance and salability. If you’re thinking that a little re-cutting could shrink a culet to a normal size, be wary. A flat pavilion and large culet could mean heavy weight loss.