The precious rock, described as an "outstanding vivid blue diamond with extraordinary saturation, tone and clarity," is the biggest of its kind found at Cullinan, which Petra controls since 2008.
Based on current prices of about $2 million per carat, the freshly unearthed gem value could easily exceed $60 million.
Last year, another exceptional 25.5 carat blue diamond was found in the same mine, and sold for almost $17 million.
Diamonds from both the Cullinan mine in South Africa and the Williamson facilities in Tanzania, both owned by Petra, have been displayed at London's Buckingham Palace and are regarded as among the rarest and most valuable in the world.
The 1905 Cullinan Diamond has been 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.
Petra shares went up 0.08% on Tuesday to stand at 122.60p each.