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