Evaluating the color of rubies
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Rubies with different saturation levels.
Courtesy of Renée Newman

Ruby grading chart by GemePrice

  Evaluating the color of rubies

By Renée Newman, GG (Guest blogger)
March 11, 2013

Even though it's debatable as to what are the most valuable ruby hues and tones, gem dealers agree that pure, vivid colors are far more desirable than dull, muddy, brownish colors.

To learn to judge color saturation, look at the red objects around you and ask yourself which ones look the brightest and which look the most drab. You can also go to a jewelry store and look at some rubies and red garnets side by side, and try to determine which ones have the least amount of brown and gray. Just being aware of color purity and intensity will increase your sensitivity to it. This in turn will help you choose a more desirable ruby. Keep in mind that the color of the gems in books and on computer screens are probably a little different than their actual color.

Judging the tone (lightness or darkness) of faceted gemstones is sometimes difficult because they don't display a single, uniform tone. They have light and dark areas which become more apparent as you rock the stones in your hand. To judge the tone of a faceted ruby, answer the following questions:

1. What is your first overall impression of the tone? Use words such as "dark" and "medium dark," but keep in mind that the tonal boundaries of these terms can vary from one person and grading system to another. For rubies, medium to medium-dark tones are preferred by the trade. In western cultures, corundum with a light red tone is called pink sapphire and its value is determined in relation to other pink sapphires, not to rubies. Medium tones with better saturation are priced higher than lighter tones. Usually the lighter the color the less intense the color saturation and the lower the price.
Sometimes you can see small flashes of pink in red corundum. These are called pink overtones and are considered desirable by some dealers. Other dealers, however, say that straight red rubies fetch a higher price than those with pink secondary colors.

2. Do you see nearly colorless, washed-out areas in the ruby? This is an indication of weak color, poor cutting or both. Shallow-cut stones tend to have a weak color in the center and a stronger color around the rim.

3. What percentage of the ruby looks black? If more than 90% of it is blackish, gem dealers would classify it as undesirable. The purpose of owning colored stones is to see color, not black. The GIA refers to the dark black or gray areas seen through the crown of faceted gems as extinction. The amount of extinction you see depends on the tone, the cut, the amount of red fluorescence, the type of lighting and the distance of the light from the stone. Light-colored, shallow-cut stones normally show less extinction than those which are dark-toned or deep-cut. As the light source gets broader, more diffused and/or closer to stones, they display less extinction and more color.

Judging the hue of a ruby is just as difficult as judging the tone. The different tones and possible brownish tints are distracting. Moreover, rubies are a blend of two colors--purplish red and orangy red, red and orange, or purple and red. When you look at rubies from different directions while moving them, you can sometimes see these two colors. This is due to an optical property called dichroism, whereby light is split into two different colored rays which are polarized at right angles to each other.

When gems are cut, dichroism is a major consideration. In rubies, the purest, most desirable color is normally produced when the stone is cut and oriented so that there's only a single direction of red color through the table. This can be determined by looking through a small instrument called a dichroscope. If the stone is perfectly oriented, you will see only one color through the dichroscope. If there are two directions of color, two different colors will be visible.

Cutters always have to compromise between top color and the weight they recover from the rough in order to produce the most valuable stone. The purer the face-up color and the lower the dichroism, the higher the per carat price. But if a lot of weight is lost by orienting the stone for top color, the higher per carat cost may not make up for the weight loss.

When you judge the hue, look for the dominant color in the face-up view. What's your first overall impression? Generally, the more purple or orange a stone looks, the less it costs. The redder it is, the more it costs.

Opinions differ as to what is the best ruby hue. For example, Benjamin Zucker, a New York gem dealer and author, states in his book, How to Buy & Sell Gems, that the finest shade of Burmese ruby is a full-bodied red with a touch of orange in it. The GIA, in their Gem Reference Guide, identifies the finest quality Burmese rubies as being red to slightly purplish red with a medium-dark tone and vivid saturation. Ruby connoisseurs agree, however, that the best stones have a highly saturated red color which is intensified under sunlight or incandescent light. Top ruby color is often compared to the warm glow of a true red traffic light.

About the author
Excerpted from the Ruby, Sapphire & Emerald Buying Guide by Renée Newman GG, gemologist and author of 11 gem and jewelry books. Her books are used worldwide as textbooks, consumer buying guides, sales-training tools, and references for jewelry professionals. For more information about Newman and her books: www.reneenewman.com