Cape Town — A fabled mine near Pretoria has produced another rare blue diamond potentially worth millions of dollars.

The owners of Cullinan mine have announced that an "exceptional" 29.6 carat blue stone, the size of an acorn, has been recovered from the mine.

"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 Diamonds said in a statement.

In 2009, a seven-carat diamond cut from a 26.58 carat rough stone from Cullinan, was auctioned in Geneva for nearly U.S. $9.5 million.

Petra said the latest find was "one of the most exceptional stones" it had recovered since buying the mine from De Beers.

"Blue diamonds are among the rarest and most highly coveted of all diamonds," it added, "and the Cullinan mine is the most important source of blues in the world."

In 1905 the mine - then called the Premier Mine - produced the largest gem-quality diamond ever found, weighing 3106 carats. From it were cut the Great Star of Africa and the Second Star of Africa, both of which are now in Britain's collection of crown jewels.