In the more than half a century since GIA (Gemological Institute of America) introduced its D-to-Z system for color grading colorless to light yellow diamonds, it has become the premier color grading system in the global diamond industry. During this time, technological progress, new diamond sources, and developing markets have changed the way the industry does business. As John King, Ron Geurts, Al Gilbertson and James Shigley explain in the lead article in the Winter 2008 issue of Gems & Gemology, the GIA color-grading process has evolved to meet the industry's ever-changing needs.
This article, based on the report by GIA Laboratory's John King and colleagues, reviews the history of GIA's D-to-Z color grading system.
It started with Shipley. Although GIA did not formally grade diamonds until the 1950s, the development of a uniform standard and repeatable methodology was a priority years earlier for founder Robert Shipley and later Richard T. Liddicoat, who succeeded him as president in 1952.
Before the GIA system, diamond color was expressed in myriad confusing terms and systems. (Some, such as "River" and "Wesselton" for top color and "Cape" for deeper color, are still heard occasionally within the industry.) Grading environments differed just as much. The old dictum that color should be judged only in north daylight did not take into account that such light could vary by geographic location, weather and time of day.
With no standardized light source for diamond grading available in the early 1940s, the task of creating one fell to Shipley, Shipley's son Robert Jr. and Liddicoat.
This was difficult, given that no light source at the time could consistently approximate "north" daylight. Shipley Jr.'s experiments with various types of lighting, however, led to the GIA Diamolite, introduced in 1941, which used a specially filtered incandescent bulb that approached the color appearance of north daylight. Fluorescent lighting, the basis for all modern diamond grading environments, was just becoming commercially available and was still years away from being stable enough for such an application.
Also in 1941, the GIA team developed an early color grading unit for internal use to grade master stones. Adapted from the medical field, the GIA Colorimeter did not have electronic measuring components like today's color grading instruments. Instead, it featured a movable, transparent wedge with a graduated scale that went from colorless at the tip to the equivalent of the modern P color. Roman numerals, starting with zero, served as color grades (that is, 0, I, II, III to VI). GIA used this system to grade master diamonds for American Gem Society members. Using these master stones with the Diamolite, retailers could grade diamonds with greater consistency and communicate with one another and their suppliers about the color of a particular stone.
Evolution of the D-to-Z system. A breakthrough came in 1953, when Liddicoat unveiled GIA's terminology for diamond color grading. The system started with the letter D for colorless, because so many diamond companies had been using the first three letters of the alphabet (particularly A) for their own proprietary quality grades. At first, the new system was used only in the classroom, but many students began to look to GIA to verify their own attempts at grading - and eventually to grade the diamonds for them. Gradually, the system was adopted throughout the diamond industry.
In the 1960s, after a decade of grading diamonds commercially, GIA began to use the now much-improved fluorescent lamps and to refine the viewing environment - surroundings, distances, angles and the like - to yield more consistent results.
As the diversity of diamonds available to the jewelry industry has broadened over the years, GIA has also expanded its color grading terminology. For decades, diamonds with a light brown (rather than yellow) component seen in the lab fell at the high end of the color grading scale and so were easily accommodated by the D-to-Z grading system. With the influx of a broader range of brown diamonds from the Argyle mine starting in the mid-1980s, however, the laboratory began to see such stones in colors covering the full range of the scale more regularly. Accordingly, GIA developed a set of brown master stones for internal use. All brown diamonds below J are noted with the letter grade and a color description. K, L and M colors come with the designation "faint brown," while N-to-R colors are described as "very light" brown and S to Z as "light brown".
A special approach was also developed for grayish diamonds. In the E-to-J range, gray diamonds are assigned a letter grade, as is done with browns. Beginning with K, however, they receive a description only. For example, diamonds in the K, L and M range are graded "faint gray," and those in N to R are "very light gray."
The difficulties of grading subtle differences in hue and tone as color becomes more noticeable prompted GIA to adopt a two-grade range (O-P and Q-R, for example) below N color. Indeed, the trade traditionally has not demanded finer distinctions at the low end of the scale.
The lab also faces unique challenges when it grades fancy shapes below Q color, because the body color of these diamonds can appear several grades darker when viewed face-up. The lab assigns a grade that balances face-up and face-down appearance up to the letter Z in these cases. Face-up appearance takes precedence at Z, because it is the point of transition to fancy colors, which are always graded face-up.
Machine-Supported Color Grading. While the Institute's attempts at developing color-grading devices date back to at least 1941, it was not until the turn of the 21st century that GIA developed an instrument that yields results that correlate with those of human graders. This device (which is not available commercially) is the product of years of testing involving millions of diamonds.
A prototype was built in 1999, and subsequent adjustments brought a very close match between instrumental and visual grading. Once the unit's accuracy was tested and verified, it was put into service in GIA's labs. Since its introduction, most diamonds have been graded by combining visual observation with color measurement by the proprietary device.
While keeping the same color grades and the same standards for each of those grades, GIA has adopted new technologies to ensure consistency and efficiency. In addition to the integration of an instrument into the color grading process, other recent developments have involved the choice of the Verilux 15-watt lamp for the GIA DiamondDock, the standard grading environment for diamonds in the laboratory. GIA will continue to incorporate advances in technology to maintain its goals for a consistent color grading process that can be replicated across both time and distance.