While many of you are familiar with the way most diamonds are graded for color, not a lot of people know much about how precious gemstone varieties like ruby, sapphire and emerald are graded for their most important attribute. The visual aspect of color has governed the values of most gemstones throughout history, even during a time period when the big three colored gems were valued much more highly that colorless diamond. Entire societies would covet the rare, vermillion greens of scarce Colombian emeralds, as well as the infamous 'pigeon blood' shades of highly vivid rubies from Burma (Myanmar).
The trade has often used very descriptive terms to somehow paint a picture of a gemstone's colorful beauty. For sapphires, the term 'cornflower blue' would often be touted as the finest color shade available on the high end market. Such a term was first applied to products from the Kashmir region, back when the original sapphire mines in that area were operational.
Most trade names for gem colors were created for marketing purposes, and as you might expect- there was a degree of inconsistency between different people when trying to imagine or evaluate stones based on such descriptive names. Today, the colored stone industry still holds multiple systems for conveying the color of most precious stones. There exists a much larger degree of subjectivity when dealing with this topic, and so it is always advisable to physically view a colored gemstone of any species before making a purchase. Simply assuming a color profile based on a trade term (or even a description) may be hard to depend on. Likewise images on the internet can be easily enhanced using most modern computer programs today.
At our laboratory, while we recognize the subjectivity of grading colored gemstones, we make use of a coded color-component system popularized by the GIA back when our gemological staff were taking their classes at their campus. In this system, color could be divided into three sections- hue, tone and saturation. Hue pertains to the basic bodycolor of a gemstone. This could be a single main color such as red or blue, or it could be comprised of a dominant color and a modifying color, such as bluish green, purplish red or orangey red. For some gemstones, the hue is also what determines the gem's variety. Take the mineral species corundum for instance. Corundum is defined by its chemical composition of Al2O3, crystalized in the trigonal lattice system, however its two most famous varieties- ruby and sapphire, are defined by their true dominant hue. A gem-quality corundum specimen must have a dominant red hue to belong to the ruby gem variety for example.
This system makes use of 31 different hues, each having its own abbreviated form. Purplish red for instance, would simply be coded as pR. The modifying hue would be marked as a lowercase letter, while the dominant hue would follow as a capital letter.
Apart from hue, the next component of color would be tone. Tone governs the lightness or darkness of a gemstone's color. Rated as a number from 0 (colorless or white) to 10 (black), a stone's tone plays an important factor in the overall beauty and appeal of its color. The ideal tone values can differ between many separate gem species and varieties. The vast majority of gems maintain a general tone range from 1 (extremely light) to 9 (extremely dark) and typically have their best tier somewhere in the middle of said range- i.e. 5 (medium) or 6 (medium dark). Again, it still depends on the kind of gem though, with regards to the specific ideal tone that the market tends to receive better.
The third color component would be saturation. Now saturation is the aspect of color that governs its intensity. It is also frequently described as a number that corresponds to a descriptive adjective. Most top color gems would often get a saturation level of 5 (strong) or 6 (vivid). Such would be the way that our lab (and many others) systematically describe the saturation level of a colored gem. Many also use saturation to distinguish variety differences within a species. A green beryl of low saturation for example, would simply be called 'green beryl' and not 'emerald'. A reddish corundum with low saturation or light tone might be classified as pink sapphire rather than ruby. Such topics can be highly debated issues for many gem traders, with the subjectivity of different opinions sometimes causing clashes on where the dividing line exists between two varieties.
This is just one of the many existing color systems currently being used in the jewelry trade, and while it is likely the most comprehensive one, there is no one-single universally accepted system that absolutely everyone will recognize unanimously. Likewise two people using the same system can also have their own differences of opinion, as even the human eye can read color slightly differently (across different individuals).
Color is highly subjective with most precious gemstones, and submitting your gem to a gemological laboratory will allow you to get their unbiased opinion on the specific hue, tone and saturation profile of your gem. This may help you in formulating your own opinion about its color, or aid in counterchecking how far apart your seller's opinion was from that of a third-party assessor's.
In the latter part of the year, our laboratory has procured a very rare set of color grading reference-paddles created by collaborative efforts between the Gemological Institute of America and the Pantone color company. The three dimensional color-calibrated plastic paddles mimic the visuals of a gem, while allowing you to see color differences in much greater depth and appreciation. The 324 GIA Gemset system is widely known to be the most specialized instrumental aid in determining the color of non-diamond gemstones across the history of the jewelry industry. Unfortunately due to a combination of its very high price and the acquisition of Pantone by X-Rite, the set is no longer in production for new customers to acquire today. Laboratories would either have to make use of 'flat' reference images such as computer software created pictures or relatively inexpensive transparent comparison sheets. Both of these work well enough, but do not give the same level of comparative support for color grading when pit against a full three dimensional system such as the GIA Gemset. Whenever comparative aid is necessary, our gemological staff and graders make use of this system to help evaluate gemstones graded with our laboratory since July of this year and onwards.
