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In the first part
of this article we discussed the critical importance of diaphanity,
transparency or crystal in the evaluation of gemstones. In that
installment I suggested that “crystal” was the defining
characteristic of the very finest in gemstones and, as such, is
the true "fourth C" of gemstone quality evaluation. In
this installment the discussion turns to a close-up analysis of
the three aspects of color--hue, saturation and tone, the relationships
between these criteria, and the finer points of color discrimination
in gemstones.
Viewed face-up a faceted gemstone is far from uniform.
It is a complex mosaic, a shifting crazy quilt of color, a chromatic
jigsaw puzzle. Some facets are bright, others dull; some one color,
some another. Some portions of the stone reflect light, others appear
dark. The four C’s do apply, but they require some amplification.
In order to fully understand what we see in the face-up mosaic of
color, three additional criteria are necessary--these are: dichroic
effect, color bleeding, and primary and secondary hues.
As every gemologist knows, light behavior, i.e.
refraction, reflection and total internal reflection, are a function
of proportion and cut. One ray may enter, bend and exit through
the pavilion, while another will reflect off the pavilion facets
and exit through the crown. Other light rays will totally reflect
inside the gem, eventually exiting through the crown. The behavior
of each light ray, the length of its path, and its eventual exit
point determines what we will call the face-up mosaic of the gem.
The length of the path that light follows determines
the quantity of color we see. The longer the light path, the more
color the ray picks up in its rapid passage through the gemstone.
In addition, the ray also loses some color through selective absorption.
In a singly refractive gem, this explains why some facets may exhibit
differences in darkness or lightness (tone) or appear duller or
brighter (saturation) than other portions of the stone. In a doubly
refractive gem, the scene is made more complex by the splitting
of each ray into two components, each containing a portion of the
visible spectrum. In dichroic gems, additional hues may be added
to tonal variations, further complicating the faceup mosaic.
Dichroic effect is simply any two-color effect
visible in the face-up mosaic of the gem. Bleeding is an
old term that can be defined as a specialized negative subset of
dichroic effect. A gem is said to bleed color when it loses
saturation/tone when the lighting environment is changed from natural
daylight to incandescent lighting and vice versa. For example, a
slightly violetish blue sapphire of 80% tone bleeds to a blue of
50% tone, giving the hue a washed out appearance. When we discuss
bleeding we are talking mostly about tonal variations of the same
hue. It is common in sapphire, less so in ruby, emerald, tanzanite
and tsavorite. It is a distinct negative in the gem varieties of
sapphire, ruby, emerald and tsavorite, where the ideal is a visually
pure single hue.
Dichroic effect is more general and not necessarily
negative. Stones which show dichroic effect simply are those that
show two or more distinct hues when viewed face up. Dichroic effect
is considered a negative quality in what might be called primary-color
gem varieties, i.e. those in which the ideal color is a visually
pure single hue. In other gems such as andalusite and tourmaline,
dichroic effect can make a positive contribution to the attractiveness
of the faceted gem.
Although scientific analysis is considered objective
and connoisseurship merely subjective, science can, at times, be
useful to the connoisseur. In such a case the color scientist unknowingly
becomes an ally in the attempt to get a grip on the finer points
of connoisseurship. There are eight spectral hues; red, orange,
yellow, green, blue, violet, purple and pink.
Normally gemologists measure tone (lightness to
darkness) using a scale from 0-100%. Zero tone is equal to a totally
colorless transparent medium such as ordinary window glass. As the
tone approaches one hundred percent, the hue, whatever it is, becomes
darker and darker until eventually it becomes oversaturated, i.e.
black. Think of it as adding black paint to a bucket of paint of
any pure hue. The more black, the darker the color gets until it
eventually just blacks out.
According to color scientists, each hue achieves
its optimum saturation at a certain tone. Color scientists call
these optimum levels saturation gamuts. For example the
transparent hue blue reaches its optimum saturation at about eighty
percent tone. For most sapphire aficionados,
the optimum tonal value for a fine sapphire falls between seventy-five
and eighty-five percent. Eighty percent is the median--hitting right
on the blue gamut. It seems clear that a very close relationship
exists between a hue’s optimum saturation/tone and the preferred
tonal level for gemstones personifying that hue. This leads to a
simple, perhaps self-evident conclusion--the brighter-hued stone
is the better-hued stone.
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