Tuesday, June 2, 2015

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



14 comments :

Keystring said...

Beautiful gems, good to know how they are rated!

Jean Sandell said...

Good info!!! Sometimes I like the heavily included gemstones better because of their opaque color.

Mollie Ann said...

Always a great source of information: the JET Team Blog! Thank you!

Urban Pearl Studio said...

I would like to place an order for the Tanzanite, please
Thank you very much.

Michele said...

Interesting info, always learning something new!!!

Tracy said...

Great article! SO informative!

LoveStoneArts said...

Thanks for the info!! I've looked up appraisals for colored gems and notice as you mention, that there are different criteria on different sites. Awaiting your next installment eagerly.

SendingLoveGallery said...

good info!

jemsbyjb said...

Great to know about this.

Beadsme said...

Great info.

Gemstones on My Mind said...

Very interesting. I also like the inclusions in some stones and matrix in some beads also. Sometimes it seems as if it's grading how formal or bohemian a piece of jewelry is meant to be.

WanderingJewel said...

Very interesting info

jemsbyjb said...

Thanks for the informative articles on this topic.

Gloria Flynn - EarthEnergyGemstones said...

Great information! I, too, like some stones with inclusions or matrix, but those eye clean ones are pretty gorgeous!!