Assessing Colored Gemstones, Part 1 - The Three Cs

Have you ever wondered how colored gemstones are assessed and graded? This week, the JET Team begins a four-part discussion explaining the basics of assessing fine colored gemstones. We start by taking a look at the criteria on which grading is based. All gemstone images shown are courtesy of Heritage Auctions.

Diamonds are valued and rated with the Four Cs - color, clarity, cut and carat. Colored gemstones are evaluated similarly, but while carat weight will affect the final price of a colored gemstone, the size itself is not graded. Thus colored gemstones are evaluated only on the Three Cs of color, clarity and cut, and the final grade of a colored gemstone will take all three of these factors into account.

Left: Excellent rectangular citrine, 68.60ct
Excellent bluish violet oval tanzanite, 14.95ct
Good emerald-cut tourmaline, 3.38ct

While diamonds are always graded according to the same international criteria, there is unfortunately no universal grading system for colored gemstones. For example, AGL (American Gemological Laboratories), AIGS (Asian Institute of Gemological Sciences) and GIA (Gemological Institute of America) all have their own, different rating systems, and there are many more systems also in use across the globe.

Some suppliers describe colored gemstones using words like Excellent, Fine, Very Good and Good, but each supplier may apply the terms using slightly different criteria. Other suppliers may use comparable terminology of Extra Fine, Fine, Good and Commercial. Potential gemstone buyers need to be aware of these differences, and make sure they know which scale has been used to determine the grade of their colored gemstone.

From left: Excellent emerald-shaped aquamarine, 32.93ct Good cushion-cut yellow sapphire, 26.80ct Excellent oval amethyst, 133.52ct

The following table shows a basic comparison of the most commonly used systems used for grading colored gemstones:

From left: Very good undrilled black South Sea cultured pearl, 14x13.5mmExcellent whistle-cut rhodolite garnet, 5.22ctVery good black opal cabochon, 7.98ct