Color Grades for Brown Diamonds
Beyond the D-to-Z scale, diamonds with a distinct
brown color might earn a grade of Fancy Light, Fancy,
Fancy Intense, Fancy Dark, Fancy Deep, or Fancy
Vivid depending upon their appearance determined by
the combined effect of hue, tone and saturation. All of
these “Fancy” color grades are determined by
examining the gem in the face-up position. That
makes the color assessment more similar to what a
layer will experience if the diamond is displayed or
used in jewelry.
The Origin of Brown Color
Many brown diamonds are thought to derive their
brown color from being subjected to stress and
incredible pressure underground. This stress deforms
the diamond crystal lattice and produces a texture
known as “graining” within the diamond crystal. This
graining is sometimes visible during microscopic
examination as a series of parallel to subparallel grain
lines similar to the “grain” seen in a piece of wood.
The deformed crystal lattice changes the way that
light passes through the diamond. It causes the
diamond crystal to absorb certain wavelengths of light
and the visible light that reaches a human eye causes
us to see the diamond as “brown”.
Brown graining is typical of brown diamonds from
the Argyle mine in Australia. Pink diamonds from
Argyle receive their pink color from similar
graining.
This illustration shows how plastic deformation causes brown
color in diamond. In the illustration, each gray circle
corresponds to a carbon atom in the regular repeating network of
atoms in a diamond crystal. During its time in the Earth, the
diamond is subjected to forces that cause planes of carbon atom
displacement within the diamond crystal. This displacement
changes how light passes through the crystal, and the planes
selectively cause brown light to be transmitted. The planes are
parallel in orientation, and if there are enough of them the
diamond will have a brown color to the observer. If the crystal is
viewed through a microscope looking parallel to the orientation
of the planes, the edges of the planes will appear as parallel
brown lines. They are called “brown graining” because the
brown lines are reminiscent of wood grain.
Brown Diamonds from Argyle
In the mid-1980’s the Argyle Mine in the East Kimberly region of Western
Australia began producing diamonds. It quickly became the largest
diamond producer in the world on the basis of carat weight.
A very small percentage of the rough diamonds mined at Argyle could be
cut into colorless “white” diamonds. The mine also produced a small
number of colored diamonds in pink, red, blue and violet colors. But, most
of the non colored diamond rough coming out of their mine did not meet
De Beers’ definition of ‘gem quality’.
About 80% of the Argyle rough was brown, under 0.1 carat in size, and
most stones had clarity problems. De Beers, the company who held a nearmonopoly
on diamond rough at that time, was only willing to pay an
industrial-grade price for these diamonds so the great majority ended up as
drill bits and for use on grinding wheels and other applications.
A rough-brown diamond exhibiting an octahedral
crystal form and a color typical of the Argyle Mine.
Crystal is approximately one centimeter from the
top termination to bottom.
Courtesy: Arkenstone
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