Blue diamonds, the rarest of all diamonds, were forged billions of years ago, by tremendous pressure and heat - 2192 degrees Fahrenheit, to be exact. This kind of hellacious heat and horrendous pressure originates over one hundred miles below the earth's surface in the upper mantle. Under these conditions, carbon atoms and traces of boron come together and link to form what we know today as a blue diamond. The difference in the amount of boron, radiation and pressure on the diamond's structure determines the depth of color in a diamond. The diamonds are then brought to the surface, in a speeding tunnel of molten lava moving so fast - thirty to forty miles per hour - that they reach the surface in mere three and a half hours.
The most notable blue diamond mines are found in India and in South Africa. Although the mines in India have produced more blue diamonds than the South Africa mines, the diamonds with deepest color and best clarity are usually found in the most southern regions of the African continent.
According to the Natural Color Diamond Association, "Blue diamonds are considered extremely rare. Ranging from the pale blue of a winter's day to the deep hue of a peacock's tail, each shade is exquisite". Unlike clear diamonds where clarity and cut determine a diamond's worth, a blue diamond's value is determined first by color, particularly depth of color and then clarity. Blue diamonds are so rare, for every ten thousand clear diamonds, there is only one true blue diamond in existence.
The "unluckiest" diamond, the Hope Diamond was once owned by Marie Antoinette. Known for it's signature blue color, this diamond weighs 45.52 carats. Legend has it that originally the Hope Diamond weighed over one hundred carats until, in 1669, King Louis XIV of France, bought the stone, had it cut down to 67 carats and named it "the French Blue". Currently, the Hope Diamond is on display at the Smithsonian Institute in Washington DC.
Like other blue diamonds, the Hope Diamond is a type II-B diamond, a classification used to describe diamonds that are semi-conducting, meaning that they can conduct an electrical current. Other diamonds, clear or colored, can not do this. According to the PBS website, Treasures of the World "...when you take the Hope Diamond into a dark room and expose it to ultraviolet light, then turn off the ultraviolet light, the diamond will glow a deep ember-orange color. A very few other blue diamonds have been known to show the same orangey (sic) phosphorescence".
The "largest" polished blue diamond, the Idol's Eye, weighs 70 carats. Originating from the Golconda region of India, it possesses what gemologists call a "a very distinct delicate blue color", often referred to in older gem literature as a "rosy blue". Legend has it that the diamond once set in the eye of a temple in Benghazi, Libya and shortly after WWII, the diamond was sold to an American aristocrat named Mrs. May Stanton. Rumor had it that Mrs. Stanton, who lived alone in a palatial mansion, wore the Idol's Eye while she ate breakfast alone every morning.
The most famous blue diamond, the Heart of the Ocean, better known as "Le Coeur de la Mer" became a star on it own merits when it was featured in the James Cameron's blockbuster movie "Titanic".