The rare 37.3-carat stone, which the auction house was hoping would entice bids of up to $30 million, was among several lots that did not find buyers at the sale in Geneva on Wednesday, in a possible sign of a softening in the diamond market.
Sotheby's had high hopes after rival Christie's on Tuesday sold a huge 163.41-carat diamond known as the "Art of Grisogono" for nearly $34 million.
Although that was the highest price ever paid at auction for a D colour diamond, an industry expert said it fell short of expectations.
The top price ever paid at auction for a Fancy Intense Pink, one of the rarest and most highly sought after of coloured diamonds, is $46.2 million paid for the "Graff Pink" in 2010.
Other lots that failed to find buyers on Wednesday included the "Donnersmarck diamonds", two yellow gems weighing 102.5 and 82.5 carats that once belonged to the Paiva, a Parisian courtesan of the 19th century.
Tobias Kormind, head of 77 Diamonds, a major European online diamond jeweller, said earlier this week he was disappointed by the hammer price for the "Art of Grisogono".
"This is a worrying sign for the top end of the diamond market suggesting it is not as immune as one would have hoped to downward price pressures caused by global factors such as the Chinese slowdown, Russian sanctions and lower oil prices," he said in a statement.