Cut

This is the only characteristic that has been dictated by man.  The proportional balance of a diamond is very important to maximize the sparkle of a diamond.  The way a diamond sparkles is called Fire and Brilliance. This is the one factor that has the greatest influence in the brilliancy of the stone.  Rough diamonds are of uneven shapes when they are mined, the expert cutter studies the grains and structure of the stone to determine what type of cut is best for that stone.  For a diamond to make the best use of light the cut should have ideal proportions.

Light is captures when it touches the diamond's surface.  Some light is reflected back into the eye of the observer (known as external reflection).  The rest of the light travels through the stone and if the diamond is proportionally correct, it will be deflected towards the center of the diamond (known as refraction) and then back towards the crown (known as internal reflection).  The brilliance of the diamond depends on the amount of external and internal reflection of light.

If a diamond is poorly, or if an inappropriate shape was chosen for the raw stone, the light either escapes through the bottom or is deflected through the sides, lessening the diamonds brilliance and fire.

After much experimentation, the "Round Brilliant" cut was found to be the most effective at creating fire and brilliance, while minimizing the weight lost from the original stone.