Extremely rare and can fetch prices of over $1 million per carat.
Author: Hobart M. King, Ph.D., GIA Graduate Gemologist
What Are Red Diamonds?
Red diamonds are the rarest variety of colored diamonds. In the entire world, only a few diamonds with a pure red hue are found in an entire year. The primary source of those red diamonds has been the Argyle mine in the East Kimberley region of Western Australia, which is scheduled to close in 2020. The color of most red diamonds is caused by glide planes in the diamond crystal, along which carbon atoms have undergone slight displacement.
How Rare Are Red Diamonds?
Red diamonds are so rare that between 1957 and 1987, no diamonds with a pure red color were graded by the Gemological Institute of America.  The GIA lab grades more diamonds than any other lab in the world, and the fact that no pure reds were submitted for grading over a 30-year period is a strong testament to their rarity.
The Argyle Mine, the leading producer of red diamonds, came online in December of 1985, and that is when a few red diamonds per year started appearing at the GIA laboratory. Since then, the Argyle mine has produced at least 90% of the world’s red diamonds.
Slightly less rare are diamonds with a modified red color. Modifying colors include brown, purple, and orange. These produce diamonds that are brownish red, purplish red, and orangy red.
The diamond market may have an even more limited number of new red stones entering the market, because the Argyle Mine is expected to close in 2020. At present, no new sources of even occasional red diamonds are known.
What Causes the Red Color?
The Argyle mine is located in an area of Australia that was subjected to the compressive forces of the Proterozoic Halls Creek Orogen. About 1.8 billion years ago, an ancient continental collision compressed the rocks. These forces are thought to be responsible for dislocating carbon atoms in many of Argyle's diamonds. 
The high temperatures and shear stress of the plate tectonics are thought to have resulted in plastic deformation in the diamonds. The deformation is a slight displacement of carbon atoms along glide planes parallel to the octahedral direction of the crystal.
These planes of displacement influence how light passes through the diamond and can cause the selective absorption or selective transmission of certain wavelengths of light. Most often, these glide planes cause a selective transmission that produces brown diamonds.
Less often, the glide planes cause the selective transmission of red light. When there are few glide planes, a small amount of red light transmission produces an apparent pink color of the diamond. Light red diamonds are called "pink" diamonds during grading. Fancy Vivid pinks might appear to be "red" to many observers; however, the strict rules of color grading designate them as "pink". Only in very rare situations are enough glide planes present to produce a more intense saturation of color, resulting in a rare and wonderful Fancy red diamond.
Pink Diamonds Are Light Red
Many people do not realize that pink diamonds and red diamonds both have a red color. The difference between "red diamonds" and "pink diamonds" is one of color intensity. During the color grading process, a diamond with a weak to moderate saturation, accompanied by a light to medium tone, will be called a "pink" diamond. The name "red" is reserved for diamonds with a strong color saturation and a medium to dark tone. The name "Fancy red" is given only to those diamonds that exceed the saturation levels of Fancy Vivid pink and Fancy Deep pink.
Many people who are unfamiliar with the procedures used to grade colored diamonds would, at a glance, call a diamond with a very light red color a "pink diamond". Most humans have been conditioned since childhood to use the name pink for objects that have a very light red color. Many of those same people would, at a glance, think that a Fancy Vivid pink or a Fancy Deep pink was a "red diamond". The grading is stricter than many people expect. The best way to comprehend this is to study the color reference charts for colored diamonds published by the Gemological Institute of America. 
In the colored diamond grading system, the name "red" is used so sparingly that only a very few diamonds have an intensity of red color that is able to earn it. A person could hold the opinion that the rarity of red diamonds is more a matter of "grading" than a matter of "color".
A similar use of "red" and "pink" in gemology is in the grading of gem corundum. Corundum with a vivid red color is called "ruby", while corundum with a light red color is called "pink sapphire" or "fancy sapphire". The difference in price between a "ruby" and a "pink sapphire" can be significant. As a result, submitting a gem for grading can be accompanied by anticipation and apprehension.
Cutting to Optimize Red Color
To maximize the amount of red light transmission, red diamonds are often cut with their tables parallel to the color-producing glide planes. This maximizes the amount of light intersecting the glide planes as rays of light descend through the table and down into the diamond. Much of this light reflects from pavilion facets and travels back up toward the table, passing through those same glide planes a second time to accumulate more color.
The work of the person who plans and executes the cutting of these diamonds is very important. Proper planning and cutting is important for all colored diamonds, but it is especially important when cutting red. Proper cutting can produce a more saturated and even color when the diamond is viewed in the face-up position.
Famous Red Diamonds
The Hancock Red
One of the first colored diamonds to grab newspaper headlines was the Hancock Red. Its sale in 1987 for $880,000 ($926,315 per carat) was an amazing price for a 0.95-carat diamond.
The Hancock Red had been around for a while. It was cut from Brazilian rough, and one of its first owners was Mr. Warren Hancock, a Montana rancher who was also a collector of colored diamonds. Mr. Hancock paid $13,500 for the gem in 1956. At that time very few people were interested in colored diamonds, and most people had never thought about them.
The 1987 sale occurred a few years after Mr. Hancock's death at a Christie's auction in New York City. The red diamond sold for $880,000, a price of $926,000 per carat - eight times its pre-sale estimate. That was the highest price per carat ever paid for a gemstone of any kind, producing a 6500% profit for Mr. Hancock's estate. The spectacular price brought enormous media and celebrity attention to colored diamonds.  
The Moussaieff Red
At 5.11 carats, the Moussaieff Red is the largest red diamond known. The rough used to cut the Moussaieff Red was found by a farmer in an alluvial deposit in western Minas Gerais, Brazil. The rough weighed 13.9 carats and was acquired by the William Goldberg Diamond Company in the mid-1990s.
They cut the rough into a triangular brilliant known as The Red Shield. The diamond was purchased by Shlomo Moussaieff in the early 2000s and is now owned by Moussaieff Jewellers Ltd. Sale prices for the diamond have not been revealed. 
|Red Diamond Information|
 Red Diamonds - The Rarest of Them All: Article on the Gemological Institute of America website; accessed September 2019.
 Natural-Color Pink, Purple, Red, and Brown Diamonds: Band of Many Colors: by Sally Eaton-Magaña, Troy Ardon, Karen V. Smit, Christopher M. Breeding, and James E. Shigley; Gems and Gemology, Winter 2018, Volume 54, Number 4, pages 352 to 377, 2018.
 Colored Diamonds: Color Reference Charts: by John M. King (editor); a .pdf document published on the Gemological Institute of America website.
 The Hancock Red Diamond: by Benji Margolese, article on the Leibish.com website, accessed November 2019.
 An Important Exhibition of Seven Rare Gem Diamonds: by John M. King and James E. Shigley; Gems & Gemology, Summer 2003, pages 136 to 143.
 Changing a Diamond's Color: Article on the Gemological Institute of America website; accessed September 2019.
 Diamond Treatments and What They Mean to You: Article on the Gemological Institute of America website; accessed September 2019.
 Two Treated-Color Synthetic Red Diamonds Seen in the Trade: by Thomas M. Moses, Ilene Reinitz, Emmanuel Fritsch, and James E. Shigley; an article published in Gems and Gemology, Volume 29, Number 3, pages 182-190, Fall 1993.
Red Diamond Produced by Treatment
Red diamonds have been produced by treating diamonds of other colors in a laboratory. Irradiation followed by heat treatment or annealing has successfully altered the color of some diamonds to red. Thin films applied to the surface of diamonds have successfully been used to produce diamonds of all colors, including red.  
Synthetic diamonds have also been treated to a red color. In 1993, two red diamonds were submitted to the Gemological Institute of America's Gem Trade Laboratory in New York City for a standard "origin-of-color" report. GIA gemologists identified the stones as synthetic diamonds, and the red color was determined to be consistent with post-growth irradiation and heating. An article about these treated synthetic diamonds was published in Gems and Gemology. It was one of the first published reports of synthetic diamonds that were identified to have post-growth color enhancement. 
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