A poorly cut diamond will actually loose light and appear dull. There are two main parts to the make of a diamond. Proportions and Finish. Proportion refers to the angles and relative measurements of a polished gem and the relationship between them. A diamond s proportions are not as obvious as its size and shape, but are important to the overall appearance.
Proportions determine the optical qualities of the diamond.
The GIA and the Gem Trade Laboratory state that the table size, crown angle and pavilion have the most dramatic effect the optical qualities of a diamond. Finish is the quality of the polish. It is the overall condition of the facet surfaces of a finished diamond. Good polish is the essential for maximium optical qualities of a diamond This information was taken from the Jewelers of America, Inc., and the Gemological Institute of America.