If diamonds are romantic, then blue diamonds are swoon-worthy. Naturally blue diamonds are extremely rare, and significant specimens are found only once in a blue moon.
Why are you blue? How nature makes a blue diamond.
Most of the time, the coloration of a blue diamond is caused by boron. The more boron that is present, the richer the color of blue -- although it is important to note that a blue diamond would rarely achieve the vividness of a blue sapphire. Usually blue diamonds have a grayish cast. If you happen to see one that seems green-blue, that diamond has likely been exposed to radiation. That doesn't necessarily mean that the color has been altered in a lab situation; radiation exposure can happen in natural environments as well. Hydrogen exposure can also influence the color of diamond. The result tends to be in the gray-violet to gray-blue range.
Some history, mystery, and intrigue behind blue diamonds.
Just like white (or colorless) diamonds, blue diamonds were first discovered in India.
One mysterious blue diamond is the 70.21 ct Idol’s Eye, which historians believe was found in India’s Golconda district sometime in the early 17th century. Spinners of tales used to claim that it was plucked from the eye of a statue. This is a common myth surrounding coveted gemstones. In this case, it is only a story and not fact. In 1865, the Idol’s Eye was sold to an undisclosed person at Christie’s in London -- and by 1906, it was known to be in the possession of Abdul Hamid II , the 34th Ottoman Sultan. Where is it now, I wonder?
You can't talk blue diamond lore with touching upon the Hope Diamond. In the 17th century a French gem dealer sold King Louis XIV a big, wonderful blue diamond. My goodness, but that stone had some adventures. Once it arrived in Europe, it started its career as the Tavernier Blue. Later it was recut and renamed the French Blue and was incorporated in the French crown jewel collection. During the French Revolution, the crown jewels were stolen and many of the precious stones in that collection were cut into smaller specimens so that they could be sold -- undetected. It is thought that that the Hope Diamond might actually be the French Blue, just recut. Henry Philip Hope bought the Hope Diamond in 1930. (Of course, it wasn't named that until after he bought it!) Many legends about the curse of this blue diamond have circulated in the twentieth century but for now it rests safely at the Smithsonian Institution.
While India has been historically the source for blue diamonds, recently significant stones have been found in various South African mines and occasionally in Borneo. However modern blue diamonds will never (probably) match the older stones in mystery and romance.