"This spectacular gemstone was recovered on Thursday 24 September and is currently with experts for analysis," said a statement released by the London-listed company which operates mainly in Africa.

Initial examinations of the diamond which weighs just over 100 grams (3.5 ounces), have shown it to be of exceptional colour and clarity.

The diamond is undergoing colour grading, but is believed to be a rare Type II diamond, with very low traces of nitrogen -- considered an impurity -- making them among the most transparent and colourless of the gems.

"The Cullinan mine has again given the world a spectacularly beautiful and important diamond... we now eagerly await the findings of the expert analysis," said Johan Dippenaar, Petra's chief executive.

The precious stone was found alongside three other special white gems of similar colour and clarity, a large diamond of 168.00 carats and two other stones of 58.50 and 53.30 carats.

The mine which was previously owned by diamond mining giant De Beers, is renowned for the discovery of the famed Cullinan Diamond in 1905, which is part of the British crown jewels and weighed 3106 carats.

In May 2008, the mine produced a sparkling 101.27 carat diamond, roughly the size of a ping-pong ball.

The Cullinan Diamond Mine is the third richest diamond producing mine in South Africa.