Diamonds come in all shapes and sizes, from impressive pink jewels to rare ancient black diamonds thought to have formed in outer space. The rarest and most prized of all these diamond types is the elusive red diamond with only 20-30 specimens thought to exist in the world.
Mostly found in South Africa, Australia, and Brazil, the rarest of all the colored diamonds get their color from an unusual process during their formation. Unlike other colored diamonds that get their hues from chemical impurities such as nitrogen within the diamond structure, true red diamonds are pure carbon. While rubies are a famously red gemstone they get their color from corundum and the element chromium, not carbon.
The exact science of this process remains a hotly debated mystery within geology circles. According to Diamond Rocks London, some believe that the red color comes from a process called plastic deformation, whereby the pressure under the surface of the Earth changes the molecular structure of the diamond. Coupled with this is the idea of deformities in the soil that contribute to the red color, according to The Diamond Pro.
As a result of this process, most red diamonds are quite small, only around half a carat to a full carat in size. Despite this, they are some of the world’s most expensive, with prices of around $1 million dollars per carat as typical. These 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 largest red diamond in the world, known as the Red Shield or the Moussaieff Red, was discovered in Brazil in 1989 and sold for $8 million in 2021. The Red Shield weighed 5.11 carats and was graded a Fancy Red.