All diamonds are special, but it’s the big ones that really grab everyone’s attention. Consider, for example, the 6-carat pink engagement ring that Jennifer Lopez received from her erstwhile fiancé Ben Affleck a few years back. That marriage never happened, but the engagement ring remains enshrined in the memories of diamond lovers, despite the fact that it was soon replaced by a larger diamond from her new husband, Marc Anthony.

Of course, just because a diamond is large doesn’t mean it’s worth the Earth: there’s a huge qualitative difference between the Hope Diamond and a fist-sized chunk of brown, cloudy bort that’ll end up at the end of a drill-bit someday. Then, too, you sometimes have to look closely to determine whether a stone is indeed a diamond; just because it sparkles like one doesn’t mean it’s not moissanite or a zircon. The differences between a big diamond and a big fake are often clear only to a trained eye. Similarly, it takes an expert to spot hidden flaws and evidence of artificial enhancement, or to differentiate natural stones from laboratory-produced ones.

Even if a stone is inarguably a gem-quality diamond, the variation among the grades is often tiny, and seldom obvious to the uninitiated. For that reason, there’s a critical need for services like AGS Laboratories (an offshoot of the American Gem Society), which provides unassailable documentary proof of diamond quality. AGS Labs, founded in 1996, grades diamonds on the basis of its own special standards, which it regards as the highest in the industry. AGS is willing to grade a diamond of any size for anyone who can afford it, but it’s built most of its business by grading diamonds for major retailers and handlers all over the world. If you’ve ever bought a real stunner at a major jewelry store, and it came with paperwork detailing the stone’s quality, type, size, color, and light-scattering properties, it’s a good bet that the reports came from AGS Labs or one of its competitors.

For some reason, clients tend to be most anxious about their larger stones, particularly those stones over three carats. In the first week of January 2007, AGS Labs responded to client concerns by establishing an enhanced service system, whereby the grading of large diamonds is expedited free of charge. For stones over 7 carats, the expedited grading takes just one business day; for diamonds of 3-6.99 carats, the wait for grading results is three business days. Obviously, AGS Labs knows how to take care of its best customers.

In it early years, AGS Labs performed proportion-based diamond assessments based on two-dimensional measurements of diamond properties. However, in 2005, AGS switched to a performance-based cut system that examines not just the stone as a whole but also the individual facets, all in a three-dimensional measurement system. In fact, their Performance-Based Cut Grading System, released in 2005, was the first such grading system for fancy cut diamonds; it was specifically developed for round brilliant and princess cuts.

Aside from the knowledge of its graders, the most important tool in the AGS Lab arsenal is light. AGS uses sophisticated ray-tracing techniques to determine the path, quality, and quantity of light that it shines through the diamond under study. The final grade of the diamond is based on factors that include brightness, dispersion, leakage, contrast, and weight ratio, in addition to proportion, symmetry, and polish.

Ultimately, even though two diamonds may look identical to the naked eye, they may well end up being graded entirely differently. One may be graded as DIF — Colorless, and Internally Flawless — while its twin may get a designation of JVVS, or Near Colorless, Very Very Slightly Imperfect. They may not seem different to the casual glace, but you can bet there’s a big difference in price.