The bottom part of the diamond, between the cutlet and the girdle.
The largest facet at the top of the diamond.
The two main parts of the diamond that need to be considered is the pavilion and the crown. Their relationship with one and other is what determines how well the diamond sparkles.
So, what happens when the light hits a diamond that has been cut well? Well, some of the light will immediately reflect back off the crown angles while the rest of the light will enter the diamond. When it enters the diamond, it should bounce off the internal walls and then shoot back to the surface of the diamond again, creating a stunning sparkle. If the diamond has not been cut well, the light will escape.