These are extremely rare. Blue diamonds will not have the deep blue color that you find in sapphires. Generally, they will be a lighter shade of blue and have a hint of grey. The blue color itself is caused by Boron impurities within the diamond. The higher the percentage of Boron, the deeper the blue color.
If you are looking for the most rare and exotic stones on Earth, then look no further than a Fancy Color Diamond. Diamonds are created under the most extreme conditions. To alter the color of a diamond, a small number of additional elements must interact with the carbon atoms as the diamond is created.
It is estimated that there is a 1 in 10,000 chance that a diamond can be created naturally with any color on the spectrum. For instance, less than one Boron atom in a million carbon atoms can alter a diamond’s color to blue.