They’re very very rare, it seems

https://en.wikipedia.org/wiki/Red_diamond?wprov=sfla1

A red diamond is a diamond which displays red color and exhibits the same mineral properties as colorless diamonds. Red diamonds are commonly known as the most expensive and the rarest diamond color in the world, even more so than pink or blue diamonds, as very few red diamonds have been found. Red diamonds, just like pink diamonds, are greatly debated as to the source of their color, but the gemological community most commonly attributes both colors to gliding atoms in the diamond’s structure as it undergoes enormous pressure during its formation. Red diamonds are among the 12 colors of fancy color diamonds, and have the most expensive price per carat. They will typically run in the hundreds of thousands of dollars per carat range. Since they are the rarest color, it is difficult to find them in large sizes, and they are mostly found in sizes less than 1 carat. Red diamonds only exist with one color intensity, Fancy, although their clarities can range from Flawless to Included, just like white diamonds. The largest and most flawless red diamond is the 5.11 carat Fancy Red Moussaieff Red Diamond, which has internally flawless clarity.