Those who remember high school mathematics should know what a parabola is. One of the many interesting things about a parabola is the focal point. Reflective surfaces made in the shape of a parabola will reflect light through this focal point, meaning all light reflected will come out of a parabola in a straight beam. This is the principle behind headlight and torches. A more complex version of this occurs with light travelling through a well cut diamond.

An Ideal cut diamond will refract any light thought the front face of the stone. This is a similar concept to the parabola, but very different in execution. Light reflects inside a parabola, but only light entering from the front; no light can travel through the back of the object. Diamonds on the other hand, will refract light as well as internally reflect it. As such, even light from behind an ideal cut diamond will find its way to the front. The sparkle of a diamond is due to this refraction and internal reflections- the maximum amount of light finds its way to the front of the diamond.

Of course this only works with an ideal cut diamond. The ideal cut was mathematically designed to display this property, and it only works with the refracting properties of a diamond.

Why aren’t all diamonds idea cut?

The most valuable diamonds are ideal cut so that all the light, all the sparkle, comes through the front face. The one major disadvantage with this approach is that a little over half of the diamond is left on the cutting room floor. The buyer is paying for the parts of the raw diamond that were not used. Other cuts, such as the deep cut, will have less wasted material, but they will not reflect light and sparkle nearly as well.

Shallow cut diamonds appear larger than they really are when viewed from the front face, but have poor light reflection.

Deep Cut diamonds waste less material when cut, so are a little cheaper for a larger carat size, but they also have poor light reflection. They can look fairly impressive.

Ideal Cut is between these two extremes, with a very tight tolerance. Though costing more the light reflection makes them look impressive, which many feel is the whole point of a good diamond and a justification for the cost.