What is the rarest diamond colour?

Red colour in diamond comes from a deformation of the carbon crystal lattice during formation, not a chemical impurity. Most fancy colours involve trace elements: yellow from nitrogen, blue from boron, green from natural radiation. Red and pink are different. The crystal has to be stressed in exactly the right way as it forms; too little stress and the stone reads pink, too much and the structure cracks. Argyle in Western Australia produced more than 90% of the world's pink and red diamonds before closing in November 2020, which has pushed verified Argyle pink and red prices steadily higher in each year since.

Almost none of this matters in a typical engagement ring. Gem-quality red diamonds average around US$1 million per carat at auction, and the high-grade examples sit well above that. The rarest colour grade that practically affects buyers is D, the top of the colourless scale, which makes up a small share of mined production. Most engagement rings use diamonds in the G to J range, which read as white once set in any metal. Spending the premium for a D rather than an F or G rarely shows in the finished ring; spending it on cut grade does.