The diamond, weighing in at an impressive 25.5 carats , was recovered by Petra Diamonds.
"It's very unusual for a diamond of this quality and size to come to market," said company spokeswoman Cathy Malins.
The Cullinan mine is famous for producing blue diamonds, including the largest gem-quality diamond ever discovered, the Cullinan diamond, at 3,106.75 carats before cutting.
The Cullinan diamond was split into a number of pieces, including the second-largest polished diamond in the world, known as the Great Star of Africa.
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