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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.
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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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