History of Cut
The Science behind Diamond Cutting
The exact process of diamond cutting is deeply intervined with the physics of diamonds, namely their optical properties. The beauty of cut diamonds stems from the interaction of light with the diamond crystal. There are four phenomena that contribute to this beauty, all of which produce a different effect and often require conflicting proportions in a cut stone to create the maximum effect:
- External brilliance — luster produced by light reflecting from the surface of the diamond. The amount of light reflected is related to the angle of incidence of light rays, and is relatively low for diamonds, only becoming significant at above 70° with respect to the vertical. This means that the reflections from the table are very small, making it transparent, while crown facets can exhibit this type of brilliance to a higher degree.
- Internal brilliance — light refracted and reflected from the inside of the diamond, mainly the pavilion facets. Refraction is a term used when light bends its path while passing between two media of different optical density — media in which light travels at a different speed, such as air and diamonds. It is described quantitatively in terms of index of refraction. This value is 1 for air for all practical purposes and 2.42 for diamonds. The difference in the indexes of refraction is so great, that a phenomena known as total internal refraction often occurs. Total internal refraction is the bending of light backwards to keep it traveling within the same medium. In diamonds, it happens when the incident angle is greater than 24°. When the precise angles are calculated for several consecutive refractions, it is possible to redirect the light coming from the top of the diamond to exit through the top again, to the eyes of the observer, making the diamond look very bright. The less light is reflected from the surface of the diamond upon incidence, the more of it is available for refraction. That is why a well cut diamond will be optimized to refract as much of the light coming through the table as possible, since this is where most light will be coming from.
Dispersive brilliance — fire and splitting of colours after the light bounces off the inside surface of the diamond several times and emerges as a rainbow. This effect is due to the small differences in the indexes of refraction of different colours of light. Different indexes cause the light to be refracted at slightly different angles, causing a ray of white light to separate into a rainbow that widens as the light goes through multiple refractions. It is important to note that light will only bend upon exiting the inside of the diamond at an angle other than 90°. This means that coloured flashes are almost always produced by light coming from the crown and rarely by light coming from the table.- Scintillation brilliance — the changing pattern of light and dark visible when the diamond is moved, caused by light reflecting from different facets. The more facets the diamond has, the more pronounced is the effect as light moves quickly from facet to facet when in motion. Although a standard brilliant cut contains 58 facets, this number is only optimal for diamonds in sizes between 0.1-5 carats. When a diamond is too small, the resulting facets will be too thin to be easily resolved by human eye, so an eight-cut containing only 16 facets may be used for a better overall look. At the same time larger diamonds may look too dull with the standard number of facets, which is why almost all significant large diamonds are custom cut to increase scintillation. The 284-carat Centenary diamond, for example, has a total of 247 facets.
A specific cutting style and shape is a compromise between all the possible combinations and it takes cutters a lot of experience to know what would be the best cut for a specific stone. A notable diamond fashioner, Gabriel Tolkowsky, has been commissioned to cut many exceptionally large rough stones, and he had to devise a special cut for every one of them. He considers working with diamonds to be such a complex and artistic profession, that he dislikes being called 'diamond cutter', considering the wording inappropriate. The skill of diamond cutting, faceting and polishing is very difficult to master since there simply is not a single solution that fits all stones.
