The beauty of a diamond does not manifest itself only through its colour or shape. As far as the stone's beauty is concerned, the ultimate beauty of a given diamond is determined by the human touch, that is, its cut. It is the way a diamond is cut (the precision-crafted geometry of its roundness, width, depth, size and position of the facets) that determines the stone's brilliance and fire. An ideal-cut diamond is a stone that is cut and polished to ideal overall proportions which perfect its optical properties, in particular, the high index of refraction and colour dispersion. So even if the colour and clarity are perfect, the poorly cut diamond will be dull and less impressive to the eye, because the light that enters the stone, will reach the facets and then leak out from the sides or bottom of the diamond instead of reflecting back to the eye.
Most gemologists agree upon the following diamond cut grading: Poor & Fair, Good, Very Good, Premium and Ideal Cut.
A diamond rated as fair reflects only a small amount of the light that enters it. Usually, these diamonds are cut to maximise the carat weight over other considerations.
These are diamonds that reflect a fair amount of the light. Their proportions are far from being ideal, because just like poor-cut diamonds they are created to keep their largest possible size rather than to mesmerise you with their optical characteristics. Diamonds from this range are the perfect option for customers who want to stay within a budget without compromising on quality or beauty.
These diamonds create a good deal of brilliance, reflecting most of the light that enters them. These diamonds are the cutter's middle ground, when he tries to stick to the necessary proportions whilst slightly sacrificing them to create a larger diamond. This means that lots of the parameters of diamonds from this range overlap with certain parameters of diamonds from the Premium and Ideal ranges. Commonly, these diamonds are priced a bit lower than Premium cuts.
Excellent (Ideal Cut)
These are the top quality diamonds. Producing maximum brilliance and fire (light dispersion), ideal-cut diamonds cannot help mesmerising the onlookers. This category includes only round diamonds.
Keep in mind, however, that cut is only one of the factors you should consider when buying a diamond, and it's highly unlikely for the casual observer to discern the difference in the proportions between an ideal cut and a poor cut.