The 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.
The angles and finish of any diamond are what determine its ability to handle light. A good cut gives a diamond brilliance (the brightness that seems to come from the very heart of the diamond) and scintillation and dispersion (the sparks of color that seem to leap out from the diamond's surface when the diamond is tilted back and forth). [Don't confuse Cut with Shape. Cut refers to how well the diamond has been designed and proportioned by the cutter. As you can see in the images below, when a diamond is well-cut, the light that enters through the table travels deep into the pavilion. Once light reaches the pavilion, it bounces back and forth on the mirror-like inside walls of the pavilion, multiplying in brightness and intensity before reflecting back out of the diamond through the table and to the observer's eye.
To understand how the pavilion works to make a diamond so brilliant, think of an empty room that has a single candle as its only source of light. If a mirror is added to the room, the mirror will reflect the image of the candle, creating a second image of the flame and essentially doubling the amount of light in the room. If more mirrors are added to the room, the brightness in the room will increase the number of reflections of the original candle flame increases. A well-cut diamond works in much the same way as a series of mirrors that multiply the brilliance of the original light source. This light is the brilliance we mentioned, and it's this effect that makes diamonds so mesmerizing.
However, when a diamond is poorly cut (either too shallow or too deep), the light that enters through the table reaches the pavilion facets at the wrong angle and fails to be intensified and reflected properly; it 'leaks' out from the sides or bottom of the diamond rather than reflecting back through the table to the eye. Less light reflected back to the eye means less brilliance. The of the most important to for in a diamond.
Diameter: The width of the diamond through the girdle.
Table: The largest facet at the top.
Crown: The top part from the girdle to the table.
Girdle: The narrow band around the widest part of a diamond.
Pavilion: The bottom part from the girdle to the culet.
Culet: The facet at the bottom tip. The preferred culet is not visible to the naked eye (graded "small" or "none").
Depth: The height from the culet to the table.
The first step to choosing a diamond is selecting from one of the major diamond shapes available. This step of the diamond buying process is one of the easiest and it is strictly dependent upon personal preference. Round brilliant stones are the most popular and are thought to be the quintessential diamond shape. It is important not to confuse diamond shape with diamond cut. One shape is not necessarily better than another is. For each shape, the cut can range from ideal to poor, meaning that the amount of refraction and dispersion of light ranges from ideal to poor.