Color in a diamond is a glittering imperfection. Diamonds are carbon, locked tightly into a lattice structure, a three dimensional grid, that makes them the hardest natural substance. Diamonds are crushed into shape deep underground, in inconceivable conditions of temperature and pressure that make them dense and impervious.
They are also incredibly pure. Nitrogen, the commonest gas on earth, is the commonest imperfection in diamonds, appearing in 99% of them. However, there is only about one nitrogen atom for every million carbon atoms in a diamond. Boron, nickel, and hydrogen also appear in diamonds, also as a minute addition to an overwhelming mass of carbon.
Nitrogen, in the right configuration, can make diamonds yellow, though it often has no effect. Boron makes blue or steel-gray diamonds. Hydrogen and nickel can also color diamonds, perhaps blue, violet-blue, or violet.
Pink, red, and brown diamonds are not tinted by impurities, or not solely by impurities, as far as anyone can tell. Their color seems to have a different source. It appears that the color of these diamonds comes from an actual deformation in their internal structure. The lattice has holes, or perhaps it has ripples.
Misalignments in the lattice rupture the bonds between the carbon atoms. After the diamond grows, likely during its journey to the surface, the lattice is pushed out of shape, warped. (This, incidentally, makes pink diamonds more exacting to cut and polish.)
Red and pink colors arise when the energy of the light that hits the stone is absorbed by these deformities. The diamond is not absorbing the pink, of course, it is absorbing everything else, and reflecting back enchanting shades of champagne pink, rose, or raspberry.
The Argyle Mine
More information about pink diamonds can be found at the Argyle Mine website. They admit that the color of a pink diamond is something of a mystery, and they should know, they produce more of them than any other mine in the world.
In fact, the Argyle mine in Western Australia produces more than 90% of the pink and red diamonds found every year. Even there, they are rare. The mine processes a million carats of rough diamond for every carat of pink or red diamond they sell.
The Argyle produces the largest volume of diamonds in the world. They bring diamonds up into the light, and cut and polish them. Nevertheless, the real source is 100 miles below the surface of the earth, where pink diamonds are born.