This might well be the most frequent question a gemmologist—gem professional—can get in his work. The answer is simple.

Diamond is a mineral, gemstone, pure carbon as for chemical composition, with atoms arranged in cubic crystal system.

Brilliant is the most common and most widely used cut how cutters form rough diamonds. The cut has 58 facets and girdle.

Even other gemstones different from diamonds can be cut in the round brilliant shape, however, the optical effect and visual quality of such combination would never reach the level of round brilliant diamonds. The most important reason is that brilliant (or brilliant cut) is based on the mathematical study of light performance in diamonds, elaborated by the father of modern brilliant cut—a cutter from a Belgian diamond cutter family—Marcel Tolkowsky in 1919. Tolkowsky's two-dimensional mathematical model made an attempt to derive the "best" proportions (table size, pavilion angle, and crown angle) for a round brilliant, using physical properties of diamond material. Tolkowsky argued that certain values for these three proportions produced "the most fire and greatest brilliancy." Though these findings were not the final answer, they influenced the diamond cutting for decades.

Besides the brilliant cut, diamonds are processed in different shapes too. The most common cuts include oval, pear, heart, princes, marquise, emerald, and many others. These cuts are however much more rare compared with brilliants. The primary reason is less efficient use of diamond optical dispositions than with round brilliant cut.