Cut is by far the most confusing of the 4 Cs, since it can refer to the cutting style, the shape of the stone (round, square, heart-shaped, etc.), its proportions, or the workmanship of the actual diamond-cutting process. Each of these four characteristics are important while evaluating a stone, so we will discuss each separately.
Diamonds, as crafted by nature, consist of translucent crystalline carbon. If the outer rough could be peeled away, the resulting stone would be as smooth as glass; unfortunately, it would be no more aesthetically pleasing than a piece of glass crystal. The art of the diamond-cutter is to transform that chunk of crystallized carbon into a beautiful piece of jewelry.
The diamond-cutter has two basic types of cuts in his arsenal: The step cut and the brilliant cut. You may also hear of hybrids of the two basic cuts, such as the emerald cut, or you may also encounter some of the older-style cuts, such as the rose cut, the old European cut, or the old mine cut. If you are planning to purchase a diamond for a modern engagement ring, you only need to concern yourself with the two basic cuts.
The step cut has parallel facets that usually span the length or width of the stone. Refer to the picture shown here, and notice the "steps" that lead from the outer edges to the top of the diamond. If a step-cut diamond has rounded-off facets in the corners, this is a variant of the step-cut called the emerald cut.
The brilliant cut has triangular facets that surround the stone and usually culminate on a flat top called a table. Again, refer to the picture shown here and notice how the triangles fit into each other. The modern and popular brilliant-cut round engagement diamond has 58 of these triangular facets -- 33 above the middle of the stone (or the girdle), and 25 below.
One point of clarification is that you cannot just go to the jewelry store and purchase a generic step-cut or brilliant-cut diamond -- you must select a stone with a given shape, that will in turn be created using step-cuts, brilliant-cuts, or a mix of the two. Continue with the tutorial to learn more about diamond shapes.