Professional Jeweler Archive: A Fancy for Treated Color

October 2002

Diamonds/Gemology


A Fancy for Treated Color

Not natural, but more affordable


Natural fancy colored diamonds can make strong men swoon – from their beauty and their price. In fact, natural colored diamonds – especially saturated and larger than a carat – often end up at major auction houses to be snatched up by a handful of collectors.

They can be so valuable that gem labs issue make-or-break reports. The slightest nuance in a color grade can cost hundreds of thousands of dollars per carat.

Enter Treatments

Diamonds that are treated to accentuate or change color are another matter. Because of the vast price difference between natural color and treated, it’s important for a qualified lab to be the final arbiter of color origin. The treated colors in diamonds you’ll likely see are in the green-to-blue and red-to-yellow ranges.

Greens and Blues

Green diamonds are caused by radiation whether the color is natural or induced. In nature, diamonds lying within radioactive environments turn greenish, particularly their exterior. Often these diamonds lose the green after cutting. Radiation treatments using high-energy electrons generally create deeper, more penetrating colors than those found in nature. Greenish and bluish diamonds irradiated in a lab often show uneven color zoning, one indicator of treatment. Determining color origin using common gemological equipment is difficult to impossible; labs should be contacted to make the determination.

Natural blue or grayish blue diamonds (Type IIb) owe their color to an impurity: boron substituting for carbon atoms, and are rare in nature. Type IIb stones are electrically conductive, while irradiated stones are not. Treated blue diamonds do not owe their color to boron as a trace element but to radiation produced in a laboratory. More recently, pale blue diamonds have been made through high-pressure/high-temperature techniques, which change the perceived color of certain diamonds – the diamond’s crystal lattice changes the selective color absorption. These treated diamonds are rare and expensive.

Reds and Yellows

Reds (including pinks), yellows and related hues modified by purple and orange generally are caused by irradiation followed by controlled annealing. The reddish colors are very rare, even as treated diamonds, partly because the types of diamond needed for the color change are rare. The treatment generally occurs in an oxygen-free environment to prevent the diamonds from burning.

In nature, rare reddish diamonds are thought to owe their color to plastic deformations of the crystal. Treated reds and pinks exhibit strong orange short-wave fluorescence, though natural pink and red diamonds do not. Natural reds and pinks often show uneven color concentrations (primarily along grain lines), while the color is more evenly distributed in treated diamonds.

Some rare natural pinks with low nitrogen content owe their color to nitrogen vacancy centers. According to The Nature of Diamonds (Cambridge University Press, 1998) some Type Ib treated pinks also get their color this way. After irradiation, vacancies are created and, if followed by annealing, can become trapped with isolated nitrogen atoms, creating the nitrogen vacancy centers and causing the diamonds to be perceived as pink. Irradiation and annealing also produce unusual purplish or violet shades that are very rare in nature.

Natural yellow diamonds (Type Ia) get their color from nitrogen impurities. Treated yellow diamonds also contain nitrogen and are Type Ia in general, though irradiation results in more saturated colors.

Synthetic diamonds are increasingly being perfected to look like natural yellow or treated colors. Telltale microscopic and fluorescent clues, which once clearly pointed to synthetic origins, are much less evident in some newer synthetics, say gemologists. Larger diamonds whose color origin or identity is suspect should be tested by a qualified laboratory.

– by Robert Weldon, G.G.

From left: 1.04-ct. irradiated and annealed red diamond, 1.08-ct. irradiated and annealed yellow, 1.06-ct. irradiated green and 1.11-ct. irradiated and annealed blue diamond. Courtesy of Nice Diamonds, New York City; (800) 536-6423.

Photo by Robert Weldon.

Fancy intense to fancy vivid yellow diamonds with HPHT enhancement. Courtesy of By Nature LTD, New York City, (212) 575-0657.

Photo by Robert Weldon.

Copyright © 2002 by Bond Communications