Author: Hobart M. King, Ph.D., GIA Graduate Gemologist
What Are Yellow Diamonds?
Yellow diamonds are diamonds that have an obvious yellow bodycolor when viewed in the "face-up" position. 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 a specific 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 a faceted diamond is viewed in the face-up position.|
Below 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 colored diamonds are 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.  (This reference is for the GIA "Color Reference Charts". If you have a strong interest in colored diamonds, you really need to see these charts.)
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 secondary color. Greenish yellow and orangy yellow are 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 secondary 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 crystal lattice by strong covalent bonds. When they are composed of pure carbon and are without inclusions or structural defects, they are colorless.
Nitrogen atoms are very small and have the ability to substitute for the carbon atoms in diamond's crystal structure. Trace amounts of nitrogen substituting for carbon in the diamond crystal lattice will cause the diamond crystal to selectively absorb blue light and selectively transmit yellow. This will cause those nitrogen-bearing diamonds to have a yellow color. 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. One way that influences color is when a single nitrogen atom is shared by four carbon tetrahedrons. This defect is known as the "C center" and is shown in the accompanying illustration. In this configuration, just one nitrogen atom per 100,000 carbon atoms can produce a noticeable yellow color in the crystal. 
This type of nitrogen substitution produces yellow color in lab-grown diamonds but not in mined diamonds. During the long history of natural diamonds, nitrogen atoms usually aggregate into clusters of two or more nitrogens, sometimes combined with a vacant lattice position.
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. 
Some types of nitrogen substitution in diamond do not produce yellow color. An example is 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.
A nitrogen defect that produces yellow color in many mined diamonds is the N3 defect. It consists of three nitrogen atoms clustered around a vacant carbon position in the diamond crystal lattice. When an N3 defect is accompanied by an N2 defect, certain wavelengths of blue and violet light are selectively absorbed by the diamond, and yellow light is selectively transmitted. This gives the diamond an apparent yellow color in the eye of the observer. 
"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, many of these diamonds would be light enough in color that they would receive a color grade within the D-to-Z color scale; however, some would be graded as "fancy-color diamonds". The name "Cape" is still used today by many diamond professionals 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-grade, 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.
The Design and Color of Settings
When buying a yellow diamond for use in jewelry, the color of the metal used in the setting can be important. The first thing to consider is the contrast or harmony of colors that will be present when the diamond is viewed in the setting. The color of the metal can contrast with the diamond and make it stand out in the setting; or, the color of the metal and the color of the diamond can be in harmony with one another. You will have choices of gold, platinum, rose gold and other metals, each with a unique appearance. Your jeweler can provide valuable advice and possibly show examples to inform your decision.
The second thing to consider is how reflections of the metal band and prongs will influence the appearance of the diamond. Light reflected from the metal can enter the diamond and reflect from facet to facet to facet throughout the diamond. The color of the setting can have a noticeable impact on the apparent color of the diamond, especially when its tone and saturation are light. Again, your jeweler can be an important source of advice on both the color of the metal and the design of the setting.
Yellow Diamonds and Treatment
Yellow diamonds have been produced by treating brownish diamonds. These treatments include HTHP (high temperature, high pressure), irradiation, annealing and coating. Some of these treatments can be reversed or altered if the diamond is subjected to heating 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 similar diamonds with a natural color. The buyer should also be informed of any special care requirements. Many buyers have no interest in treated diamonds. They want gems with a natural color and are willing to pay a premium for them. At the same time, some people are glad to buy a treated stone because it enables them to get a larger diamond for the same price or the same size diamond at a lower price.
|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 in the early days of lab-created diamonds.
Today, producers of lab-grown diamonds have better controls over the diamond growing process and are able to exclude nitrogen or introduce it in just the right amount to produce a predictable yellow color. Yellow lab-grown diamonds are available and are priced at a significant discount to mined yellow diamonds.
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