The cut of a diamond determines how a diamond shines, the cut does not refer to the shape of the diamond. The master diamond cutter has created the facets on the diamond to allow light to internally reflect from one mirror-like facet to another and disperse and reflect it through the top of the stone in a display of brilliance and fire. So when we talk about diamond cut we are referring to the reflective qualities of the diamond which determines its ability to handle light, which leads to brilliance.
When a diamond is well-cut, light enters through the table (top of the stone) and travels to the pavilion where it reflects from one side to the other. If the diamond is cut poorly like in the diagrams below, where one is cut too deep and one too shallow, the light will escape from below. But when the cut is ideal, such as in the example on the right, the light reflects back out of the diamond through the table and to the observer’s eye. This light is the brilliance we mentioned, and it’s this flashing, fiery effect that makes diamonds so mesmerizing.