The Girlfriend's Guide to Jewellery
21: Cut 2011-06-30
Whatever the clarity or colour of a diamond it is the cut that brings a diamond to life! Most people don’t need a microscope to appreciate the brilliance of a diamond or to notice its lack of brilliance. Either way, it is the cut that can give a diamond the fire and sparkle we all love. The temptation to retain greater finished weight from the rough crystal can cause a cutter to deviate from the ideal proportions when cutting a diamond thus diminishing the fire and brilliance of the finished stone.
The cut grade of a diamond has three components: proportions, symmetry and finish. An ideal grade in all three categories is the finest possible of cut grades. It is a grade very rarely achieved and requiring a great deal of time and the skill on part of a master cutter.
What we know today as the round brilliant cut has been developed over centuries in an ongoing search for the form to release the maximum brilliance and dispersion from rough diamond material. The round shape brings symmetrical uniformity and intensity of brilliance to a diamond. The correct angles and proportions maximize the return of light to the eye.
While it is part of the fascination of cutter’s art to find ways to deviate from the round shape and still maximize a diamond’s brilliance and dispersion ultimately deviation from a round shape will allow light to ‘leak’ from the stone.
The angle formed by the crown and the pavilion is called the ‘critical angle’. We call it critical because it creates to angle at which light will enter and exit the diamond and it dictates the shape and proportions of the rest of the stone. The only dimensions not dictated by the critical angle are the crown height and table size.
As light moves from one medium to another, from air to diamond in this case, it bends or refracts. For the most part, the light we are interested in is the light that will ultimately return to your eye after it exits a diamond.
picture: Marcel Tolkowsky
Most of this is the light that originally enters and exits through the table and crown facets. As light enters a material it refracts at a predicable angle. By cutting the crown facets of a diamond to the so-called critical angle we can control the path the refracted light will follow. After entering through the crown of the diamond the refracted light will then reflect off the interior surfaces of the stone from one side of the pavilion to another and eventually exit the diamond. When the critical angle is correct, all of the various surfaces outside and inside the diamonds will work together to capture the light that enters the stone. The precise angles and placement of these surfaces allow the captured light to play inside the diamond without leaking out. This gives the diamond brilliance. When the captured light ultimately exits the diamond it refracts or ‘bends’ one last time. Exiting the diamond through carefully positioned and angled crown facets causes the light to break up into the beautiful, lively, fiery prismatic play of colour we call dispersion. This dispersion gives the diamond fire
The size of the crown facets is determined by the size of the table. When more light leaves a diamond through larger crown facets it makes for a much more fiery display. A larger the table makes a thinner the crown and smaller crown facets. Smaller crown facets make for less dispersion; a less fiery exit of light from the stone.
A very well cut diamond costs more but it’s worth it!