Petra Diamonds have discovered a rare blue 25.5 carat blue diamond at its famous Cullinan mine in South Africa, that could be worth more than $10 million (£6.5million).

Rare Blue Diamond

The diamond is a rare blue colour and Petra says that the stone is considered to be a 'high quality gem diamond of top colour'.

Blue diamonds are among the rarest and most highly coveted of all diamonds and the Cullinan mine is the most important source of blue diamonds in the world. Since Petra acquired the mine, it has produced a number of blue diamonds, including a 26.6 carat rough stone which was cut into a fancy vivid blue, internally flawless 7.0 carat stone. This important and rare diamond sold for $9.49 million (or $1.35 million per carat) at a Sotheby's auction in May 2009, being at the time the highest price per carat for any gemstone sold at auction and the highest price for a fancy vivid blue diamond sold at auction. It was subsequently named the 'Star of Josephine' by its new owner.

One of the most famous diamonds to be discovered at the Cullinan mine was the 'Great Star of Africa' in 1905, and is the 2nd largest diamond in the world. The Pear Shape diamonds weighs 530.20 carats and is mounted in the head of the ‘royal scepter'and can be viewed as part of the crown jewels on display at the Tower of London.