Author: Hobart M. King, Ph.D., GIA Graduate Gemologist
What Are Yellow Diamonds?
Yellow diamonds are diamonds that have an obvious yellow bodycolor. The yellow color is usually caused by small amounts of nitrogen contained within the diamond’s crystal structure.
Yellow diamonds with a rich, pure yellow color are the world’s most valuable yellow gemstone . Many people consider them to be the most beautiful yellow gem because of their brightness, fire, and exceptional luster.
Yellow is the second most common fancy color in diamonds, with brown being the most common. Yellow diamonds are found at many diamond deposits throughout the world. They are not unique to a specific geographic area or diamond mine. 
Isn’t Yellow Color a Bad Thing?
Nearly all gem-quality diamonds are graded on a color scale that most highly values the absence of color. The most widely used color scale for grading diamonds is the D-to-Z color-grading scale developed by the Gemological Institute of America (GIA). 
On the GIA scale, a grade of "D" or "colorless" is the highest grade. Traces of yellow, brown, gray, or any other color within the gem will cause it to receive a lower color grade on the D-to-Z scale.
|To answer the question... Isn’t yellow color a bad thing? Not always. Yellow diamonds are a wonderful thing when the yellow color is strong enough to be obvious and attractive when the diamond is viewed in the face-up position.|
At grade "Z" at the bottom of the GIA grading scale, a very small number of diamonds have a yellow color that is strong enough to be obvious and attractive when viewed in the face-up position. These colorful diamonds are very rare and highly desirable. They are graded on a special scale according to the strength and quality of their yellow color. These diamonds are said to have a "fancy" yellow color. On a scale developed by GIA, they are graded as fancy light, fancy, fancy intense, fancy dark, fancy deep or fancy vivid according to the tone and saturation of their yellow color. 
Pure Yellow and Modified Yellow
The most desirable color for a yellow diamond is pure yellow. However, most yellow diamonds have at least traces of a second "modifying" color. Greenish yellow, orangy yellow, and brownish yellow are all common modifications of yellow color in diamonds.
Although pure yellow is the favorite, many people enjoy the modified colors and are happy to get them at a lower price than a similar-size diamond with a pure yellow color. Greenish yellow is the more common modified color; however, orangy yellow is the more desirable and more costly. 
The Role of Nitrogen in Yellow Color
Diamonds are composed of carbon atoms, held tightly together in a crystalline network by strong covalent bonds. Diamonds composed of pure carbon and without inclusions or structural defects should be colorless.
Nitrogen atoms are very small and have the ability to substitute for the carbon atoms in diamond's crystal structure. Nitrogen is the most common impurity that substitutes for carbon and can comprise up to 1% of a diamond on the basis of mass. 
Nitrogen can exist in the diamond crystal lattice in a number of ways. The one that influences color the most is when a single nitrogen atom is shared by four carbon tetrahedrons as shown in the accompanying image. In this configuration, just one nitrogen atom per 100,000 carbon atoms can produce a deep yellow color in the crystal. 
When nitrogen atoms substitute for carbon in the configuration described above, it produces a defect in the diamond crystal which alters how light passes through. The defect causes a selective absorption of blue light. The remainder of the spectrum is transmitted, and that results in the perception of yellow color in the eye of the observer. 
The most common nitrogen substitution in diamond occurs when a pair of nitrogens substitute for carbons in two adjacent carbon tetrahedrons, with one of them shared by four tetrahedrons. This configuration is shown in a second accompanying illustration. Here, the nitrogen has almost no impact upon the color of the diamond.
"Capes" and "Canaries"
Two common names used for yellow diamonds are "Capes" and "canaries". The name "Cape" originated in the late 1800s when many diamonds with an obvious yellow color were being produced from mines in the Cape Province of South Africa. They were quickly noticed in the marketplace by diamond professionals who began calling them "Capes" because of their Cape Province provenance.
If graded today, most of these diamonds would be light enough in color that they would receive a color grade within the D-to-Z color scale; however, a small number would be graded as "fancy". The name "Cape" is still used today for diamonds of a light yellow color, regardless of their provenance.
"Canary" is a name used in the gem or jewelry trade for diamonds with an obvious, usually fancy, yellow color. The name is imprecise because it is used for yellow diamonds that might be in the D-to-Z color scale, or all the way up through the fancy colors to fancy vivid yellow. The name also does not imply any specific provenance. Most canary diamonds are Type 1b diamonds, colored by single nitrogen atoms substituting for carbon.
Cuts and Settings Influence Yellow Color
Setting a yellow diamond in a gold setting can make some diamonds appear to have a stronger yellow color. Light transfers some of the color of the gold setting into the gem. On the other hand, the same diamond in a white metal setting can make the diamond appear to have a lighter yellow color. Bright reflections from the setting cause the gem to have a lighter yellow appearance.
The cut of a diamond can also influence its face-up color. Cutters experienced with colored diamonds can often recut a diamond into a shape that improves the depth of color or minimizes zones of weak or gray color. One shape that frequently performs well with yellow diamonds is the radiant cut. Other cuts that leave more weight below the girdle can deepen the color of a yellow diamond. Cutters who have the knowledge and skill required to improve the color of a light or fancy diamond can make a lot of money.
Yellow Diamonds from Treatment
Yellow diamonds have been produced by treating brownish diamonds. These treatments include HTHP (high temperature, high pressure), irradiation, annealing and coating. Many of these treatments are fragile. All are sensitive to heating of the diamond as might occur during jewelry repair. Coatings are often thin layers of silica applied to the surface of the stone. These can be damaged by abrasion, chemicals or heat.
Yellow diamonds that have obtained their color through treatment should always be disclosed and sold for a lower price than diamonds with a natural color that are of a similar size and appearance. Treated diamonds are of no interest to some buyers. Others buyers strongly prefer diamonds with a natural color and are willing to pay a premium for them. At the same time, some people who would like to have a yellow diamond are unable to afford one, but the low price of a treated diamond presents a buying opportunity.
|Yellow Diamond Information|
 Secrets of the Gem Trade, The Connoisseur's Guide To Precious Gems; second edition; by Richard W. Wise; Brunswick House Press; 404 pages; December 2016.
 Characterization and Grading of Natural-Color Yellow Diamonds; by John M. King, James E. Shigley, Thomas H. Gelb, Scott S. Guhin, Matthew Hall, and Wuyi Wang; Gems & Gemology, Volume 41, Number 2, pages 88-115, Summer 2005.
 Diamond Color Chart: The Official GIA Color Scale; article on the Gemological Institute of America website; accessed September 2018.
 GIA Colored Diamonds, Color Reference Charts: by John M. King (editor), first edition, Gemological Institute of America, 16 pages.
 Nitrogen, A Major Impurity in Common Type I Diamond: by W. Kaiser and W. L. Bond; Physical Review, Volume 115, number 4, page 857; August 1959.
 Gems Made by Man: by Kurt Nassau, Gemological Institute of America, 191 pages, 1980.
Lab-Created Yellow Diamonds
Many of the first attempts to produce diamonds in a laboratory resulted in diamonds with a yellow color. Consider that one nitrogen atom per several thousand carbon atoms can impart an obvious yellow color in a diamond. Then consider that nitrogen is the most abundant gas in the air of the laboratory. It was very difficult to keep the nitrogen out of the diamond growing process.
Today, producers of lab-grown diamonds have better controls over the diamond growing process and are able to produce colorless and near-colorless diamonds routinely.
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