A 122.52-carat blue diamond has been discovered in the South African mine where the world’s largest gem was unearthed more than a century ago.

Petra Diamonds Cullinan said the “exceptional” jewel was found in the Cullinan mine east of Pretoria, which has supplied stones for Britain’s crown jewels.

“The rarity of a blue diamond of this magnitude sets it apart as a truly significant find,” the company said in statement, adding that the gem has not yet been valued.

London-listed Petra said in January it had unearthed a 29.6-carat vivid blue stone, described as being “in a class of its own”.

Blue diamonds are some of the rarest jewels in nature, and Petra claims that its Cullinan mine is the “world’s most important source” for the precious stone.

In 1905, the world’s biggest diamond – the 3106-carat “Cullinan” – was discovered there, and later cut into two pieces to be included in the British crown jewels.

A 5.30-carat deep-blue diamond last year sold for STG6.2 million ($A11.3 million) at a London auction, setting a new world record for price per carat.