Chemically pure and structurally perfect diamonds are transparent and have no hue. But natural diamonds are very rarely pure or perfect, especially gemstone sized natural diamonds. Natural diamonds actually occur in a wide array of colors, determined by the defects and imperfections of each particular diamond. Diamonds can be found in gray, white, yellow, orange, red, pink, purple and green. The value of a diamond varies dramatically according to its color. If a diamond has a yellow hue, its value will decrease, while pink, blue and red hues will make a diamond significantly more expensive. Red diamonds are the most expensive colored diamonds.

On a chemical level, naturally occurring blue diamonds have the common nitrogen impurities found in 98% of natural diamonds. Yet blue diamonds also have high levels of boron within the crystal matrix, the absorption of which causes them to absorb light in the red, orange and yellow spectrums, often resulting in a gray or blue hue. Blue diamonds are also semiconductors. Boron impurities only occur in 0.1% of all natural diamonds, which explains why the value of pieces of jewelry made with blue diamonds is so high.

The Hope Diamond, for instance, found in the Smithsonian Museum, is the biggest and most well-known blue diamond in the world, weighing in at 45.52 carats. The Transvaal Blue is another one of the world’s most famous diamonds. It’s a 25 carat pear-cut diamond that was found in South Africa, in the Premier Diamond Mine in Transvaal, and was once owned by Baumgold Bros.

Apparently, the soaring value of blue diamonds has resulted in major diamond thefts. In the year 2000, a diamond robbery was attempted at the Millennium Dome in London. Thieves tried to steal 12 of the most expensive diamonds on the globe, 11 of which happened to be blue diamonds, and one of which weighed 203 carats.

Today, researchers have discovered various ways to produce blue diamonds artificially. However, such methods do not work on most diamonds. In these processes, diamonds are irradiated, which causes their color to modify, and then heated to stabilize their new hue. Nonetheless, the color a stone will possess after treatment is not certain.

The proportion and cut of a diamond greatly influences how well a blue diamond’s color will be brought out, which in turn has an effect on the diamond’s value. The way a diamond is cut affects how light will reflect off the different facets. The color grade of a well cut blue diamond can be two or three times greater than a poorly cut diamond of the exact same chemical quality. The cut is thus even more important in colored diamonds than it is in white diamonds, in which cut and reflection only affect the brilliance of a diamond. 

The same goes for diamond ’spreading’. Diamonds are often ’spread out’ to appear larger. In white diamonds, this damages the sparkle of a diamond, which is bad enough, and one should be cautious of falling for them due to their size alone, but this method is all the more harmful in colored diamonds. Spreading a blue diamond or any other colored diamond beyond its weight will reduce the stone’s color grade, hence decreasing the diamond’s value.