goldsmiths • jewelers • gemologists



Gemstone Connoisseurship;
The Finer Points, Part II


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Consider Paraiba Tourmaline, the saturation success story of the 20th century. Discovered in 1989, and mined out by 1990, here is a gem that has escalated in price from several hundred to tens of thousands of dollars in a single decade. The key to understanding this dizzying increase in price is to be found in Paraiba’s verbal description---“neon” or “electric blue”. Few gemstones achieve the intense saturation of Paraiba tourmaline. Perhaps the only contender, Burma Ruby, not surprisingly sells for a similar price.

Unfortunately natural things rarely come in visually pure hues. For this reason connoisseurs divide hue into primary, secondary and sometimes tertiary components. For example, a ruby is never pure red; it is either violetish, pinkish or orangish red. The ish refers to the secondary or modifying hue. Secondary hues are also pure spectral hues or modified spectral hues such as purple and pink.

This approach to grading borrows liberally from The

American Gemological Laboratory’s Colorscan grading system. The Colorscan system works well because it treats the non-spectral hues gray and brown not as hues, but as saturation modifiers or masks; the presence of either acts to dull and muddy, that is, to reduce the saturation of those hues that are present.

Among the fine points of connoisseurship in gemstones is the question of preferred secondary hues. Is a pinkish ruby preferable to a violetish ruby? This is a question that is often asked but seldom answered. In the December issue of JCK, Gary Roskin identifies pink as the preferred secondary hue in ruby2 . This is a courageous call, and Roskin deserves credit for having the intestinal fortitude to render an opinion. Unfortunately, I believe his conclusions are the wrong ones! Let’s see if an analysis of this issue can clarify a few of the points made so far.

Running the Gamut:

The hue we call pink is simply the name given to light-toned red. As with most gemstones, a pure spectral hue is preferred. That is to say the ideal color in ruby is red--the brightest, richest, purist red possible. Red reaches its optimal saturation at about 80%, a rather dark tone. When we talk about pinkish red we are talking about a secondary hue that is, by definition, a light-toned modified spectral hue, in short--a light-toned red. In ruby a preferred secondary hue is, or shou be, one that does not dilute, but rather reinforces the primary hue. In this case a light tone added to a dark tone dilutes the latter--reducing the color saturation below its optimum gamut.

This is not to say that a pinkish-red ruby isn’t beautiful. My wife has a piece I made containing a highly UV florescent pinkish red three-quarter carat Vietnamese stone that is visible across a crowded room. Lighter toned ruby, particularly florescent stones, sizzle like beef fat on a hot grill! Also, if one wishes to appeal to authority, the 15.97 carat Caplan ruby, the most expensive ruby sold to date, is a visibly pinkish gem3 .

However in ruby, pink is the least desirable of the possible secondary hues. Which is the most desirable? We are left with orange and blue. Notice I didn’t mention violet or purple. A ruby can be either, visually, but the appearance is caused by blue mixing with the primary red, which produces either a purplish or violetish hue, depending upon the strength of the blue. Remembering the spectral hues--purple is a hue-modified spectral hue falling halfway between red and blue; violet is a hue halfway between purple and blue. Thus violet is further from red on the color wheel than is purple. Whether the ruby seems purplish or violetish depends completely on the strength of the blue secondary. Given that the ideal is a pure red, and purple is closer to red, less is more--purple would naturally be the preferred secondary hue.

 

“Asking to see the pigeon’s blood is like asking to see the face of God”.

Anonymous Burmese trader

On a trip to Moguk reported in this publication I saw a few natural slightly purplish “pigeon’s blood” rubies. Because the optimum saturation/tone gamut for purple is, at 80% tone, just slightly darker than red, the purplish secondary hue visually reinforced and richened the red primary hue.

This leaves us to consider orange. Orange as a secondary hue is rarely found in Burma-type ruby. It is much more characteristic of ruby from Thailand which is formed in the iron-rich soils of Chantaburi and Trat provinces in the central part of the country. Thai rubies tend to be dark in tone with a dark gray mask that visibly dulls the red primary hue. Orange is a spectral color that achieves its optimum saturation at fairly light tonal levels, perhaps 30%. Thus a bit of orange will tend to lighten, and to some degree, brighten the overall tone.

At darker tonal levels orange becomes increasingly brownish. In fact there is really no such thing as dark orange--darker toned orange is simply brown. Brown ”muddies” the red primary hue, thus reducing its saturation. This is the reason why, with the discovery of a new source of Burma ruby at Mong Hsu, Thai ruby all but disappeared off the radar screen. Thai ruby is often of a purer red primary hue than the Burma type, but the addition of brown so reduces the saturation that the stone appears dull as dirt when compared to gems from Burma.


1_Hofer, Stephen C., Collecting and Classifying Coloured Diamonds, Ashland Press, N.Y. 1989, p.169 fig.7.7. In this illustration the author illustrates only the gamut for transparent blue. In fig. 7.13, p. 172 he illustrates the saturation gamuts for six basic opaque hues. If the left hand scale is reversed as in most gemological color grading systems, the saturation gamuts come very close to the ideal saturation/tone percentage levels in gemstones of the six illustrated hues. See also, Beesley, C.R., Colorscan Training Manual, The American Gemological Lab., N.Y., 1984, p.38
2_Roskin, G., Red Jewels, Jewelers Circular Keystone, December, 2000, pp. 71-72
3_The Caplan Ruby is a maraschino cherry red, just barely over that much disputed invisible line that divides ruby from pink sapphire. The color is similar to Lai Thai stones from Thailand. For those who would argue that the Caplan is some sort of paradigm I would like to suggest they take a look at the Hixon ruby, a large purplish red rough crystal which is in the collection of the Los Angeles Museum of Natural


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