Baku, June 14 (AZERTAC). The ‘exceptional’ 122.5-carat blue diamond was unearthed at the Cullinan mine in South Africa which is renowned for producing giant gems.

A blue stone from the mine fetched £508,000 per carat earlier this year. If this much larger one achieves a similar valuation, it would command a price tag of £62million - smashing the record price paid for a rough stone.

Anyone who buys it, however, will be hoping for better luck than the owners of the famous deep blue Hope Diamond. It supposedly puts a curse on those who possess it, as the guillotined Louis XVI and his wife Marie Antoinette discovered.

The newly discovered diamond was dug up by Jersey-based Petra Diamonds at its mine near Pretoria. One industry insider said it was almost unheard of to find a blue stone weighing more than 100 carats.

A carat is equal to one-fifth of a gram, so Petra’s 122.5-carat stone is about 25g. In comparison, the average centrepiece diamond on an engagement ring weighs about one carat and is usually white.

Shares in Petra rose nearly 8 per cent on the find yesterday, adding some £66million to the value of the company in a day.

The current record price for a rough stone was set by Petra’s sale of the 507-carat Cullinan Heritage, a white diamond, in 2010 for nearly £21million.

Blue diamonds get their colour from small amounts of the chemical element boron trapped in their crystal structure. The more ‘blue’ it is, the greater the value. They are the rarest diamonds after red, which are almost never found.

Petra’s blue diamond is being kept under guard at a secret location in South Africa and will be sold in a private tender process in Johannesburg before being cut and polished.

The Cullinan mine is recognised as the most important diamond mine in the world. It is famous for producing the world’s largest white diamond, the 3,106-carat Cullinan Diamond, which was found in 1905.

The door-stopping diamond was originally discarded by a hasty mine manager who thought it was too big to be anything other than a crystal. It was recovered and presented to King Edward VII in 1907, and gems cut from it are centrepieces of the Crown Jewels at the Tower of London.

Petra Diamonds believe the mine has another 50 years left in it.