Petra Diamonds, which operates the Cullinan diamond mine north of Pretoria, South Africa, said the find is a rare one.
"It's very unusual for a diamond of this quality and size to come to market," company spokeswoman Cathy Malins told the BBC Saturday.
A 26.6-carat blue diamond found in the mine was auctioned by Sotheby's in 2009 for $10 million.
The Cullinan mine is where the famed Jewel of Africa, a massive 530-carat pear-shaped blue diamond, was found in 1905. It was presented to England's King Edward VII and became part of the royal family's crown jewels.
The find comes at a good time for Petra, which posted a $15.2 million loss in 2012 on its mining operation, TheSouthAfrican.com said.