The acorn-sized diamond, whose colour makes it one of the rarest and most coveted in the world, was unearthed at the Cullinan mine about 25 miles north-east of the country's capital Pretoria by London-listed Petra Diamonds.The mine, which has been owned by Petra since 2008, was also where the Cullinan Diamond, the largest rough gem diamond yet discovered, was found in 1905. It weighed 3,106 carats, around 600 grams.Another blue diamond, weighing 25.5 carats, found at Cullinan last year sold for $16.9m.The latest diamond which weighs 29.6 carat blue diamond, is expected to fetch far more.Johan Dippenaar, the CEO of Petra, said it was "probably the most significant stone we've ever, in terms of blue stones, recovered"."The stone is an outstanding vivid blue with extraordinary saturation, tone and clarity, and has the potential to yield a polished stone of great value and importance," Petra added in a statement.Cailey Barker, an analyst at broker Numis, said the diamond could fetch between $15m and $20m at auction.Diamonds unearthed by Petra, which also operates mines in Botswana and Tanzania, have previously been displayed at Buckingham Palace.The 1905 Cullinan Diamond was cut into two stones – the First Star of Africa and the Second Star of Africa – and form part of Britain's crown jewels held in the Tower of London.(telegraph.co.uk)ANN.Az