Pink diamonds are a type of diamond that have a pink color. The source of their pink color is greatly debated in the gemological world but it is most commonly attributed to plastic deformation that these diamonds undergo during their formation.

Pink diamonds belong to a category of diamonds called color diamonds, the generic name for all diamonds that exhibit any sort of color. Pink diamonds range from flawless to included, as do colorless diamonds.

Numerous theories have been posited as to how the pink is formed in pink diamonds. The prevailing theory is that the pink is caused when the diamond is subjected to shear pressure during its formation. A similar theory is being tested on pink diamonds that originated in the Argyle Diamond Mine in Kimberley, Western Australia. This theory posits that a seismic shock propelled colorless diamonds to the surface and altered their molecular structure, causing them to appear pink.

Further to these theories, recent research has also indicated that pink diamonds might owe their distinctive hue to radiation exposure. As per some studies, the presence of trace amounts of radiation could cause color centers within the diamond to absorb green light, thereby creating a pink appearance. However, this theory is yet to receive widespread acceptance, and further research is required.

Another dimension to understanding the color origin in pink diamonds is through the examination of their lattice structure. It has been found that pink diamonds exhibit a more complex pattern known as "graining". This graining is thought to relate to the pink coloration, as the compressed lattice structure refracts light differently than colorless diamonds, resulting in the pink hue.

Additionally, while it's widely agreed that pink diamonds' color comes from a process called plastic deformation during their formation, the exact nature of this deformation is under debate. One recent hypothesis suggests that the pink coloration might be due to a specific type of plastic deformation that causes crystal twinning or duplication in the diamond's crystal structure.^{[citation needed]}

The same four basic parameters that govern the grading of all gemstones are used to grade pink diamonds–the four Cs of connoisseurship: color, clarity, cut and carat weight. Color is considered the most important criterion in grading a pink diamond and determining its value. However, size is an important consideration as well. The larger a pink diamond, and the better its color, the more valuable it will be.

One of the most famous pink diamonds is the Pink Star — at 59.60-carats, this type IIa diamond is the largest vivid pink diamond in the world. In 2014, the auction buyer was unable to pay the promised sum and was subsequently forced to default on its purchase. Among famous pink diamonds; the Daria-i-Noor is the largest pink diamond in the world, estimated to weigh 182 carats; together with the Noor-ul-Ain, it is the oldest known pink diamond, both belonging to the Iranian crown jewels.