Blue diamonds are diamonds with a blue bodycolor. Diamonds with a natural blue color are extremely rare, and they usually have very few mineral inclusions. [1] Their rare color and their high clarity make them extremely valuable gems.

Only a few mines produce blue diamonds, and those mines normally produce just a few blue diamonds in any given year. Their blue color is usually caused by trace amounts of boron in the diamond crystal lattice. The Hope diamond, in the collection of the Smithsonian Institution, is the most famous example of a blue diamond. (geology.com)