Cuts...
Explained
One of the important 4 C's of diamonds is the cut of the diamond. Contrary to common belief, cut does not refer to the shape of the diamond. Certainly, the cut that is chosen for the stone will have an effect on the shape of the diamond, but cut is not about shape. It refers to how the stone is cut to enhance the fire and brilliance.
The cut of a stone is determined by two important factors that make diamonds so desirable: its brilliance and its fire. Brilliance refers to how much of the incident light, that is light entering the diamond, is reflected back. It is a factor of the diamond's refractive index (RI). RI is a measure of how much the light is bent, or forced to change direction, when it moves from one medium to another – in this case from air to a diamond.
The fire of a diamond refers to its dispersive power. That is, the ability of the diamond to split light into its component colours to create a rainbow hue. It is one of the most desirable characteristics of diamonds. For fire to be at its optimum, light leaving the diamond needs to hit a facet at the greatest angle of incidence without exceeding the critical angle. The critical angle is the angle above which light is lost and not reflected or dispersed by the stone. In other words, the light needs to hit the inside of the diamond 'just right'.
So gem cutters have a difficult task ahead of them. They have to balance the external brilliance, the light that is reflected by the diamond, with the internal brilliance, the percentage of the light reflected from the pavilion – or bottom part of the stone. The goal is to aim for total internal reflection but where the light reflected does not exceed the critical angle. Not an easy task.
It is made even harder when one considers that diamonds, despite what people may think, are not all perfect when they come out of the mine. Actually, they look like rough pieces of broken glass. The choice of cut is determined by the original shape of the diamond, any inclusions or flaws the stone may have, the ideal to preserve as much of the carat as possible, and what cuts people are currently looking for.
The average diamond is octahedral – that is, it is an eight-sided object. From this standard shape, most gem cutters can cut two diamonds in the common brilliant cut. Many times, however, the diamond does not have the common shape. These other shapes are usually cut into fancy shapes for specific purposes. The problem with fancy shapes is that they are not optimised, like the brilliant cut, to show off the fire of the diamond in question. Thus, while they may have the same lustre as a brilliant cut, their fire is often reduced.
There are many different cuts available to gem cutters today that have evolved through time and with a lot of trial and error. Many of these cuts were based on the theoretical and practical work of Tolkowsky. He took fire and brilliance into account when cutting and his calculations have helped numerous cutters after him to improve upon what he originally did. The most common cut today is known as the brilliant cut.
The brilliant cut divides the diamond into two portions; the crown and the pavilion; that is separated by the girdle. The crown is important since its height and angle plays an important role in the brilliance. The facets of the head are the table that is the flat top of the gem. From the table outwards the facets are the star facets, bezel facets and upper girdle facets. The pavilion is divided into the lower girdle facets near the girdle, the pavilion facets that stretch all the way down to the culet. The culet is not always cut and was introduced to improve the structural integrity of the gem.
This is the standard cut and presents the typical 'diamond' shape that most people are familiar with. Diamonds can be cut to have round, square, heart and many other different shapes depending on what is required by the jeweller. A good gem cutter is able to cut a gem into nearly any shape while still being able to maximise the fire and brilliance of the stone.
Unless one has a lot of money to spend, most diamonds will be precut when they are bought. The cut is a very important part of the diamond's value. Even a diamond that, at first glance, appears to be of relative low quality, can be made into a sparkling stone with the right cut.
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