Diamond cut is perhaps the most important of the four Cs, so it is important to understand how this quality affects the properties and values of a diamond. A good cut gives a diamond its brilliance. The facets and finish of any diamond are what determine its ability to handle light, which leads to brilliance and fire.
Most gemologists agree that the best cut diamonds are those that follow a set formula calculated to maximize brilliance. These formulas can be seen in a diamond's proportions. If a diamond is well cut it will have maximum brilliance reflecting much of the light that enters it. If the cut is too shallow the light passes through the diamond and if it is cut too deep the light escapes through the diamond, both reflecting only a small proportion of the light that enters it.
Shape is often confused with cut. Shape is the outward appearance of the diamond, such as round, pear, marquise, etc.