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The complexities of cut |
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By trial and error, ancient jewelers produced cuts that refracted light to give
illumination life to gemstones. One of the first thought-provoking studies on how
to cut gems was published by M. Tolkowsky1 in 1919. He systematically
analyzed the optics of a diamond and estimated the best proportions for
cutting
brilliant round diamonds. With minor changes, today’s standards for
“ideal-cut” diamonds are based on Tolkowsky’s definitions.
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The round brilliant diamond shown in cross section in Fig. 1 is the best understood of all the
shapes in which diamonds are cut. It consists of 57 facets, 33 in the crown (one
for the crown table and 32 for the crown bezel) and 24 in the pavilion. Light enters
the stone through the crown and is internally reflected until it is finally refracted
and directed to the examiner’s eye or elsewhere. It is undesirable to have light
entering or leaking out through the pavilion or girdle because it will reduce the
potential of a diamond to show its brilliance. Thus the optical function of a
stone’s cut is to bring light to the observer’s eye from above the crown.
In performing this function, light is dispersed and colorful flashes
— known in the gemstone industry as fire — can be observed. An additional objective is to
create a cut by means of which a plurality of the stone’s facets are lit and others left
dark, which produces a bright appearance on the crown known as
brilliance.2 Illumination conditions and cut
proportions determine whether fire or brilliance predominates. Scintillation, another
attribute of gemstones, refers to their ability to produce an appearance of
flashes of light when gemstone, observer or light source is moved. Gemstone
beauty is a complex subject that involves the human visual system, stone cut
proportions and illumination conditions. Putting technical considerations aside for
the moment, it is indeed breathtaking to behold the beauty of a diamond that has
been optimally cut.
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©
2003 Copyright Optical Society of America
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