goldsmiths • jewelers • gemologists

Gemstone Connoisseurship;
The Finer Points, Part II

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 tone1. 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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