Judging Quality in Rubies
- Carat Weight
Of the four criteria, color is the most important factor in determining a ruby’s quality.
Objective color determination has been difficult to achieve for rubies. Ancient trade terms such as “pigeon’s blood,” “pomegranate,” “saffron,” and “China rose” have long been used to describe the color of a ruby. However, these terms are not universally recognized and an objective system for evaluating rubies did not exist until recently. Today, gemologists assess a ruby’s color based upon the three factors of hue, tone, and saturation.
The hue of a gemstone is its main color. In the case of ruby, that color is red. Secondary colors in rubies can exist as well, including orange and purple tones. The ideal color for a ruby is a vivid, medium-dark red to slightly purplish red. Many fine rubies from Myanmar have a slight purple secondary color. As a ruby’s hue becomes increasingly purple or orange, the quality of that ruby decreases.
When dealing with rubies, the lines between the color boundaries of pink and red are often blurred. Gemological laboratories usually have a set of master stones that they use as a control when assessing the color of a ruby. The ruby in question is compared to the master stones, and its color can be determined as ruby or as a pink, purple, or orange sapphire. To distinguish between rubies and pink sapphires, tone and saturation must be considered.
The tone of a gemstone is how light or dark its color is and a ruby’s tone can influence its 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 indistinct. Some Thai rubies have been said to exhibit a “garnet red” color due to their dark tone. On the other side of the spectrum, if the tone of a ruby is too light, it may be considered a pink sapphire—even if the color saturation is high.
Saturation is the third component of a ruby’s color classification and describes how pure or intense its red color appears. Rubies with high levels of saturation have more chromium in their chemical composition and can be highly saturated without becoming dark. A ruby with poor saturation would be described as brownish red. Fine rubies will either have a “strong” or “vivid” saturation.
There are other factors that may also impact a ruby’s color. If the ruby can fluoresce, that attribute can intensify the stone’s color. Rutile needles, microscopic inclusions in the stone, can reflect light and also heighten the color in a ruby.
Just like star sapphires, star rubies do exist, and can be found in all shades of red and pink. The most prized and rare color is a vivid, medium-dark red. Because they contain large amounts of silk (needed to create asterism), star rubies rarely have the bright, saturated color seen in transparent stones.
Like sapphires, rubies are not subject to the same clarity expectations as diamonds. Natural rubies and sapphires are far more rare and are not graded at 10x magnification. Rather, they are graded and viewed on eye level; a high clarity ruby would be considered “eye clean.”
For more information about clarity in rubies and sapphires, please read our Sapphire Clarity section.
Click here to learn more about the importance of cut.
Click here to learn more about carat weight.