Cover Don't miss your chance to see these incredibly rare blue diamonds in Hong Kong this December

Recovered from the famous Cullinan Mine in South Africa, these blue diamonds are among the world's most expensive

Five blue diamonds have been recovered from Petra Diamonds’ famous Cullinan mine in South Africa.

They are among the world’s most expensive diamonds, and were formed deep within the Earth—about 400 miles below the surface, which is four times as deep as white diamonds. The element boron, which is what gives the stones their blue colour, combines with carbon and crystallises under extreme pressure and heat.

The Hope Diamond is among the world’s most famous blue diamonds, and was once owned by Louis XIV (also known as the Sun King) and Marie-Antoinette, as well as George IV. Weighing 45.52 carats, it resides at the Smithsonian's National Museum of Natural History in Washington, DC and is said to be cursed, after many of its former owners suffered untimely deaths.

See also: Want To See a 117.93-Carat Rough Diamond? Head to De Beers’s Hong Kong Exhibition

The sales tender for the five Type IIb blue diamonds will close later this month, on November 24, following viewings in Antwerp, Hong Kong and New York. They weigh 25.75, 21.25, 17.57, 11.42 and 9.61 carats, respectively.

Type II diamonds contain no detectable nitrogen in their chemical structure, which means they boast exceptional transparency.

Blue diamonds are so rare that most people, even in the jewellery industry, never have a chance to see one. It’s incredibly unusual that these five diamonds were all recovered within the space of one week’s production in September.

Their birthplace, the Cullinan mine, is one of the most important sources of blue diamonds. It’s also the origin of the 3,106 carat Cullinan diamond, which was cut to form the 530-carat Great Star of Africa and the 317-carat Second Star of Africa, which are set into the British Crown Jewels.

It’s the first time that five rough blue diamonds have been offered for sale at the same time. In 2013, a 25.5-carat blue diamond was sold by Petra Diamonds for over US$16.9 million.

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