The cut of a diamond is the only one of the 4 Cs (cut, clarity, color and carat) that diamond cutters can control. The other three are determined naturally. A well-cut stone can increase the value of a lower color or clarity diamond, and conversely, a D color, Internally Flawless diamond can lose value if cut poorly. Before beginning the cutting and shaping process, diamond cutters must choose a shape; this is usually based on the natural shape of the diamond. For example an oblong stone is best suited to become a marquise or pear shape diamond while more crystal shaped stones would be better suited for a square cut.
Diamond cutting requires a great deal of patience and expertise as the diamond cutter must take into consideration the size, natural shape, cleavage points, and inclusions of the rough diamond before beginning to cut. The goal is for the finished product to be as beautiful as possible with a minimum carat weight loss.
There are four steps to the diamond cutting process. The first step is planning. Before any cuts are made, the diamond is studied, analyzed, and marked for cutting. Incorrectly marking the stone, even by a fraction of a millimeter can result in the loss of thousands of dollars. This fundamental step allows the diamantaire to understand the crystallography of the stone, finding points of fragility and tension. The planning step is key to producing the largest polished diamond possible from a rough stone.
The next step is cleaving. Diamond cutters cleave stones in order to cut them down to a manageable size. They must cleave the rough diamond at its weakest point, along its tetrahedral plane. If the diamond is cleaved at the wrong point, it will shatter, rendering it worthless.
When there is no plane of weakness, diamond cutters use the sawing technique in place of cleaving. Sawing can either be done with a phosphor-bronze blade rotating at 15,000 rpm or with lasers. At this point the diamond cutter must decide which parts of the rough stone will become the table and the girdle. The diamond is placed in cement or wax to hold it in place while the cutter carves a groove. S/he then puts a steel blade into that groove and hits it with a hammer, splitting the stone in two.
The bruting or cutting process gives the diamond its shape. When this is done by hand it is called bruting; if machines are involved it is called cutting. Whether shaped by hand or by machine, the diamond is cut using other diamonds. This process prepares the diamond for faceting.
Polishing or faceting is the last step and is comprised of three sub-steps: blocking, brilliant tearing, and polishing. The cutter places the diamond onto the arm of a rotating wheel which is covered with diamond powder. This powder smooths the diamond as it is pressed against the wheel, much like sandpaper smooths wood. The diamond first goes through the blocking stage which creates the stone’s basic symmetry. Next the diamond moves on the the brillianteering stage where the facets are polished and after which one will know how much brilliance and fire the diamond has. Minor mistakes in symmetry or proportions can cause visible differences in the stone’s brilliance.