Real black diamonds are rare. The naturally occurring black diamond is black from inclusions and differing structure. Since the clarity is one way of valuing stones, black diamonds are not as valuable, even though they are more rare. Most of the diamonds you see that are black are heat-treated low quality stones. Better quality stones are not heat-treated so they could be sold for less money than their original state. The real value is the way the stones complement the jewelry in which they are used. Black stones set off lighter settings. It then becomes a matter of taste.. less valuable than other colors, more rare than most.