Making Sense of Cut

 

July 1998

Diamonds:Gemology

Making Sense of Cut

Scintillation! Dispersion! Join the cognoscenti of diamond-cut lingo

Gem dealers, manufacturers and retailers are looking into one of the last frontiers for profit in diamonds – the "fourth C": cut.

The roles that color, clarity and carat weight play in diamond value are generally understood. The same can't be said for diamond cut. But that's about to change. The American Gem Society laboratory took the lead in including a cut grade on its diamond reports when it opened two years ago. Now the Gemological Institute of America has undertaken a major study of diamond cut, with results to be published in its quarterly journal Gems & Gemology at a future date. Some other labs around the world also have turned their full attention to the importance of cut.

What's all the fuss about cut? A diamond's cut – or "make," as some in the trade call it – transcends the other "Cs" because of the way it directs light through the diamond. Even a big diamond or one with good color can look lifeless if light enters and then leaks out because of poor cut. Customers can notice the difference, though they may not know what the difference is.

Here are some terms you'll hear in the months ahead as attention to cut intensifies and attempts to "quantify" cut characteristics are developed.

Dispersion
Dispersion is the occasional flash of color, or fire, particularly when viewing a diamond in strong, direct light. Gemologically, dispersion is the separation of white light into spectral colors, each of which vibrates at a different frequency. Dispersion flashes may be seen as any color.

Flashes of color, or dispersion.

Light Return/Brilliance
The amount of light returned to the eye, or brilliance, depends on how well the diamond in question reflects and refracts light. This includes dispersed wavelengths, which are reflected from the internal surfaces of a diamond and returned to the eye.

Quantity of light, or brilliance, returned to the viewer.

Luster
Luster is the perceived reflection and surface appearance of a polished diamond in reflected light. Generally speaking, the higher the refractive index of the gem, the higher its luster. For example, pearls and amber have lower luster and refractive indexes than diamonds or rubies.
Luster is described as metallic (the highest luster), adamantine (used specifically for diamonds), subadamantine (which refers to such gems as rubies), vitreous, subvitreous, resinous (which refers to such gems as amber), waxy, dull, silky or pearly.

The luster of a diamond, which is adamantine, can be seen in any of the polished facets. Here the diamond is at the same angle as the aquamarine, but the diamond's table facet has a brighter luster.

Refraction
Refraction refers to the bending and slowing of light as it passes at an oblique angle from a medium of one optical density (such as air) into a medium of greater optical density (such as diamond). The strength of refraction depends on the angle at which the light passes between the two and the degree to which the second medium reduces its speed.

Reflection
The return of light when it strikes a polished surface is called reflection. GIA says about 17% of the light striking the external surface of a polished diamond vertically is reflected back into the air; the greater part enters the stone.


Facet reflection on a diamond.

Scintillation
This refers to the flashes of light, or the sparkle, you see when a diamond is moved in the light. Scintillation is best when the clarity characteristics of the diamond are purest. Proper cutting angles and polish of the diamond also contribute to the diamond's sparkle.

Flashes of light, or scintillation, in a diamond.

Symmetry
This refers to the proportions that best return light and color to the eye. It's a function of proper attention to cutting angles.

Transparency
The degree to which a material transmits light and the degree to which objects beyond the material are visible is called transparency. Internal characteristics, degrees of polish and coatings all affect a material's transparency.

For more information, see the GIA Diamond Dictionary (3rd Edition), Copyright 1993 by GIA. Carlsbad, CA; www.giaonline.gia.edu.

All photos by Robert Weldon

1.01-ct Ideal Cut diamond is courtesy of David Atlas & Co. Inc., Philadelphia, PA



Copyright © 1998 by Bond Communications.


 

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