A diamond’s beauty is all about how it handles light. The most stunning diamonds reflect, refract and disperse light in a glorious display of fire, brilliance and scintillation.
But it’s no accident they do this. Diamond cutters attempt to get the best light performance out of a stone by precisely controlling its proportions and expertly angling facets to direct light in specific ways.
REFLECTION, REFRACTION AND DISPERSION
When light hits the surface of a diamond (as it does all the time unless its in complete darkness), some of it is immediately reflected by the crown facets and some enters the stone (a).
The term given to these reflected flashes of white light is brilliance. A well cut diamond has more brilliance because the angles of the stone reflect light back to the viewer, instead of allowing it to leak out of the base or sides.
The light that enters the stone is refracted by the diamond’s internal angles and is ideally directed back out through the table (b). As the refracted light exits the diamond, it’s dispersed into a spectrum (c).
Diamond experts refer to these rainbow colours caused by light dispersal as fire. It’s well documented that old cut diamonds with steep crown angles and small tables have more fire, but usually this is at the expense of brilliance. A well cut diamond will have a good balance of brilliance and fire.
Scintillation is the name given to the intense dancing sparkles created when a diamond moves. Ideally, this effect is visible across the entire surface of the diamond with no, or very few, dull patches.
When we talk about proportions, we’re referring to the measurements and angles which make up each part of a particular diamond’s anatomy and how they relate to each other.
Proportions are very important in judging the quality of a diamond because they play a big part in how light is reflected and refracted, both inside and outside of the stone. This in turn affects the diamond’s light performance - how much fire, brilliance, and scintillation it has. Diamonds with poorly balanced proportions may cause light to become trapped inside the diamond or to leak out through the pavilion where its sparkle is wasted.
A well proportioned stone will look balanced and pleasing to the eye, with a pavilion that’s not too shallow or too deep and a table that isn’t too wide in relation to the rest of the stone. If the proportions are skewed, the diamond may look lopsided.
Although the overall proportions depend on a lot more than this, the table width and pavilion depth are the ones most often used to give an idea of the overall proportions. They’re expressed as percentages making it easy to compare one diamond with another.
Because the table and depth percentages have already been taken into account to assign the diamond’s cut grade, you don’t need to worry about them too much. But it’s helpful to know about them if, for example, you want to compare two diamonds of the same cut grade, as the proportions of each will be different.
The table percentage is found by dividing the width of the table by the diamond’s overall diameter.
Table % = table width (mm)
diamond diameter (mm)
The method of obtaining the table measurement varies from shape to shape. For Round Brilliant diamonds, the longest table width is used, which is measured from point to opposite point (rather than edge to edge).
The overall diamond diameter is obtained by measuring across the girdle.
Table percentage should be neither too great nor too small. A high table percentage indicates the table is too large causing light to flood out of the diamond without being properly dispersed. Conversely, a low table percentage means the table is too small for sufficient light to get out, making the diamond appear dull and lifeless.
The depth percentage is found by dividing the depth of the stone (from the top of the table to the culet) by the diameter.
Depth % = total depth (mm)
diamond diameter (mm)
As with table percentage, the diameter is measured across the girdle.
The depth percentage can indicate how well light is refracted and dispersed by the facets. A diamond that is cut too deep or too shallow allows light to escape out through the pavilion instead of directing it back up through the table.