The universally accepted criteria used in diamond evaluation are known as the 4C's that stands for Carat, Cut, Color, Clarity. Out of the four the most important is the quality of the diamond or diamond cut;
Ideal Cut: Diamonds with ideal cut proportions are optimized to return back almost 100% of incoming light. Significant carat weight may be lost in the process of cutting to ideal proportions. Ideal cut diamonds have most intense brilliance of all diamond cuts
Premium Cut: Premium cut diamonds are almost as good as ideal cut diamonds as many people will find it very hard to tell the difference because these are fine diamonds by anybody's standards. However, there is a slight difference in quality and reduction in price over ideal cut diamonds
Good Cut: A good cut still reflects most of the light back but these diamond cuts are generally created in order to save carat weight. Some bargains can be found in this category
Fair Cut: Still reflecting good light, a fair cut is O.K. Beware of diamonds of borderline carat weight (every quarter carat) as these are optimized purely for carat weight in this class
Poor Cut: The kind of diamonds that don't make a good buy, because they appear too dull and lifeless. Typically these diamonds have been cut to maximize the carat weight over most other considerations