Loose Diamonds
The standard round brilliant cut diamond of 58 facets is cut proportionally, the diamond shows an equal distribution of light and brilliance at all distances from your eye. The center under the table is as full of light as the edge facets because the back facets are holding the light which has entered from the front.
If the diamond were cut too deep or too shallow, part of the light would pass through the back facets leaving a dark center known as a well in a deep cut stone, or a fish eye in a shallow cut stone. Some stones are naturally more brilliant than others, but may loose diamonds would be more brilliant if they were cut to better proportions.