Tuesday, June 9, 2015

Assessing Colored Gemstones, Part 2 - Colour

In this second part of our JET Team discussion about assessing fine colored gemstones, we look at the most important factor in grading the stones, color. All gemstone images are courtesy of Heritage Auctions.

Color is without doubt the most important factor in grading all colored gemstones. Of key importance is whether the colour is natural, or has been achieved through treating the stone in some way - untreated, richly colored gemstones are generally worth far more than those that have been enhanced by being heat treated or dyed. The gemstone industry has accepted methods of enhancing certain stones - for example amethysts, aquamarines, sapphires, citrines, rubies and tanzanite may be heat treated, while blue topaz is often irradiated, to improve their color.


From left: Very good emerald-cut smokey quartz, 158ct
Very good oval sapphire cabochon, 7.55ct
Excellent modified-rectangule topaz, 35.95ct


Because different stones have different characteristics, each type of gemstone has its own unique standards for optimal color and clarity. Regardless of the grading scale being used, three factors determine the grading of gemstones for color. These are hue, tone, and saturation:

  • Hue is the basic color itself, such as blue or red, and the most valuable colored gemstones will have the purest color. The GIA grading system describes 31 basic colors or hues
  • Tone refers to the lightness or darkness of color, such as dark blue or light red. The GIA system has 11 tones describing depth of color, of which medium-light to medium-dark tones are generally the most valuable
  • Saturation refers to the purity and intensity of the colour, and often determines how a gemstone will be cut. The GIA system groups colours into cool hues such as blue, and warm hues such as red, each with six degrees of saturation. The most valuable gemstones usually display strong color saturation


From left: Excellent oval natural ruby, 2.09ct
Very good rectangular apatite, 2.38ct
Good oval peridot cabochon, 18.04ct




16 comments :

Keystring said...

Excellent information here!

Michele said...

A lot of useful information!!!

Mollie Ann said...

This is my go-to place for info!

Jean Sandell said...

Good info!

TheBrassHussy said...

Beautiful gems! I'll take one of everything!

Gloria Flynn - EarthEnergyGemstones said...

As one who buys and creates with gemstones, this is excellent information. Love the beautiful examples!

SatinDollCo said...

Very informative.

jemsbyjb said...

I've learned more info which is great!

SendingLoveGallery said...

great info!

Beadsme said...

There is so much info about different phases of gems.

Ida said...

Good information! Thank you!

Tracy said...

These posts are so informative!

Jennifer said...

Thanks so much for a much needed refresher!

Ribasus said...

Great info! Thanks so much. Now... I wish I could afford that ruby. WOW.

Gemstones on My Mind said...

Fantastic info whether you're buying a gift of jewelry or choosing stones for a project! Thanks for another great article.

cooljewelrydesign said...

it's always about color ... this is a great post ... look forward to more. I always look to see how any stones are "treated." And discuss that, if applicable, in descriptions so that buyers can understand better.