When people hear the term diamond cut, their minds usually jump to the shape of the diamond. These two qualities of a diamond are completely different. The shape of a diamond can be described as cushion, asscher, pear, princess, round, heart, radiant, etc. Its cut, on the other hand, is the depth of the diamond, which allows light to be reflected in a certain way. The ultimate anatomy of a diamond will determine how radiant it appears to an onlooker. An ideally cut diamond will give it a brilliance and brightness that makes these stones so desirable. No matter the shape of a diamond, the ideal cut can be accomplished. This is why Diamond Shape is not a part of the Diamonds 4Cs.
When a diamond has an ideal cut, the light will enter through the top of the diamond, travel from side to side in the interior of the diamond, and back out toward the observer's eye. Diamonds that flash and sparkle have an ideal cut. When a diamond cut is made either too shallow or too deep, the reflection of light will leak out of the bottom or the side of the diamond. This permits less light to reflect back to the viewer, making the diamond less brilliant to behold.
The key to obtaining an ideal diamond cut is to find the perfect proportion. There is actually a mathematical formula that can be used to calculate exactly where and how to make a cut on a diamond to increase its brilliance. If you are shopping for a diamond but you want to save money by choosing a diamond without GIA certification, take some time while in the store to look at the certified diamonds, where the cut grade is known, and train your eye to recognize the greater sparkle of higher grade diamonds. Then when you return to those without an GIA certificate, you will be better able to select a diamond with a more valuable, brilliant cut.
Keep in mind that a casual observer will not be able to tell a huge difference between a poorly cut diamond and one with the most ideal cut. If you are on a budget, there is no reason why you should not settle for a diamond cut that is graded as being very good. Ideal cut (GIA Excellent Cut Grade) rank higher on the scale while good, fair, and poor rank beneath it. Diamonds with very good cuts involve jewelers straying from the ideal proportions slightly so as to create a larger diamond for you to show off and enjoy.
Even though they are not part of the Diamonds 4Cs this is the place to mention another two important criteria: Diamond Polish & Diamond Symmetry... Diamond Symmetry is an essential element of diamond finish. Symmetry means the exactness of the shape and arrangement/alignment of facets. It includes naturals, badly cut facets and additional facets, off center culets and tables, not round girdles, misalignment of crown and pavilion facets. Diamond Polish, as its name, influences on how well the light can pass thru the diamond and is essential to a diamond's brilliancy. It's recommended to choose a diamond that was graded by the GIA (or any other known gemological lab) with good, very good or excellent polish. Diamonds that have poor to fair polish are less brilliant because they have polish lines which cut back the amount of light that enters or exits a diamond. Both symmetry and polish are graded from poor to fair, good, very good and excellent - exactly like the diamond cut grading. The ideal diamond in known throughout the industry as Triple X / Triple EX which means that the diamond got Excellent grading in all three criteria - cut, symmetry & polish. A triple ex diamond gets a premium on its price which is not always worth it (and certainly not worth it if you are on a budget). You can rest assure that no one would notice if the diamond has a very good grading on either of the three parameters.
Remeber that while Diamond Cut is an extremely important part of the Diamonds 4Cs, it is not the only one. Please read about the other Diamonds 4Cs as well - Diamond Carat, Diamond Clarity and Diamond Cut.