As a colored gemstone, the quality of rubies is assessed via the same four criteria that diamonds are assessed. However, with respect to diamond evaluation, all four criteria are well defined and equally weighted. Ruby, on the other hand, lacks well defined, universally accepted quality standards and color is, without question, the most important of the four criteria.
Follow below as we explore the optimal quality characteristics for a ruby within each of the four criteria to know:
Historically, it has been difficult to describe color variation in rubies. For centuries, gem dealers were forced to rely on descriptive terms that were not universally recognized or understood. Ancient Indian manuscripts classified ruby into distinct colors such as “China rose,” “saffron,” “pomegranate,” and “partridge eyes.” In Burma (now Myanmar), the finest rubies were said to be the color of “ pigeon's blood .”
Today, gemologists seek a more universal and objective means of assessing color in rubies. Their color, and that of other colored gemstones, is now described by the hue, tone, and saturation. The description of the ideal color in ruby–vivid, medium-dark red to slightly purplish red–incorporates all three of these terms.
The hue is the gemstone’s basic color. While a ruby’s color is basically “red,” in many cases, purple or orange secondary colors can also be found. While absolute red is the ideal hue, many fine rubies from Myanmar are slightly purplish red. As a ruby’s hue becomes increasingly purple or orange, the quality is thought to suffer and the ruby loses value.
Ruby’s color boundaries are subject to considerable debate in the gemological industry. The best way to evaluate and compare a ruby’s color is with actual gemstones. Many gem labs use a set of master stones to evaluate whether corundum is true “ruby” or whether it is pink, purple, or orange sapphire. To distinguish between rubies and pink sapphires, tone and saturation must be considered.
Tone, which describes how light or dark a stone’s color is, will also influence a ruby’s value. Most fine rubies have a medium to medium-dark tone. A ruby should not be so dark that its color is obscured, nor so light that its color appears dilute or indistinct. Some Thai rubies are described as having a “garnet red” color due to their dark tone. On the other hand, if the tone is too light, the stone may be considered a pink sapphire—even if the color saturation is high.
Saturation describes how pure or intense a color appears, and it is a key component in determining a ruby’s value. Rubies with high saturation levels have more of the color-inducing trace element chromium. They can reach highly saturated hues without becoming dark in tone. A ruby with poor saturation would be described as brownish red. The very finest rubies have “vivid” saturation, but rubies with “strong” saturation are also highly prized.
A number of other factors may also contribute to the color of a ruby. The red hue of a ruby can be intensified if it has the ability to fluoresce. Inclusions can also improve the color of a ruby. Minute needles of rutile silk are highly reflective, and they scatter light within the stone, which may improve the color. A ruby’s color may also depend on how it is cut. Skilled gemstone cutters fashion rubies to maximize their brilliance , minimize color zoning , and exhibit their best pleochroic color.
Star rubies come in all shades of pink and red, although the most sought after color is the same vivid, medium-dark red preferred in the transparent faceted stones. Because they contain so much silk, however, star rubies rarely attain the bold, saturated red of the best transparent stones. Those few that do will draw high prices.
Rubies rarely exhibit the high clarity of fine diamonds. Rubies are host to many different inclusions, and even the best stones are not expected to be free of inclusions when viewed at 10x magnification. In fact, a ruby with no inclusions should be viewed with suspicion; it may be a synthetic stone or a glass imitation.
The best clarity grade for ruby is “eye-clean,” which means no inclusions are visible to the naked eye. When evaluating the clarity, experts consider the size, number, location, and overall visibility of the inclusions.
Inclusions are a natural consequence of crystal growth. Ruby inclusions vary with their source or origin and treatment history.
When a ruby with plentiful silk is cut as a cabochon , reflections from the rutile needles form a six-rayed star. Rarely, star rubies can have twelve rays, a doubling effect due to the added presence of hematite needles. Because the appearance of the star depends on silk inclusions, star rubies never achieve the degree of clarity possible in faceted stones. Nevertheless, the more transparent a star ruby, the more highly it is valued.
The condition of a ruby’s silk is also a valuable clue to the stone’s treatment history. Many rubies are heated to alter their color or enhance their clarity. The intense heat applied to treated rubies partially melts or decomposes silk. Intact silk is strong evidence that a ruby has not been heat treated, while degraded silk, recognizable under magnification by a trained gemologist, indicates that a ruby has been heated.
While a five-carat stone may be considered small for an aquamarine, a quality five-carat ruby is large enough to get the attention of serious gem collectors. As with any gem, ruby prices per carat increase with carat weight.
Fine quality rubies over one carat are rare, but commercial quality rubies are commonly available in a range of different sizes. Fine quality ruby rough is extremely expensive, so quality stones are not usually cut to calibrated sizes because it could result in a significant loss of weight. Commercial quality ruby is more likely to conform to standard calibrated sizes.
A ruby’s size, if expressed in a unit of weight, is called a carat (abbreviated “ct”). A carat is a metric unit equivalent to one fifth (.20) of a gram. One hundredth of a carat is called a point (abbreviated “pt”). A number of small rubies may be weighed together to give a total carat weight (abbreviated “tcw”). Because rubies have a high specific gravity, a one-carat ruby will appear smaller than a one-carat diamond.
The term “cut” can have several meanings when applied to rubies and other gemstones. For example, it may describe the faceting style or shape of a finished gemstone. It may also mean a gemstone’s proportion and finish. Proportion refers to the rough dimensions and overall symmetry of a gemstone. Finish describes the precision with which facets meet, the relative size and number of facets, and the quality of the stone’s polish.
Like most other transparent gems, rubies reveal their full beauty when they are cut. However, because ruby rough is so valuable, dealers and consumers accept gemstones without the precision cuts required of fine diamonds. In general, gem cutters follow four guiding principles when they fashion rubies:
At times, these guiding principles may conflict with each other. On occasion, cutters may be forced to compromise color or clarity to retain carat weight. For example, it may not be possible to orient a ruby for ideal color because the potential loss of weight would be too great. In other situations, an asymmetrical cut may be permissible because it maximizes color, conserves valuable ruby rough, and avoids highly included or fractured areas within the crystal.
Speaking of inclusions, they are incredibly important to understanding the quality of rubies, so we explore that topic next with Inclusions in Rubies.