An extremely rare, purple-pink diamond mined in Russia, which Sotheby's described as "a true wonder of nature", sold for 24.4 million Swiss francs ($26.6 million) on Wednesday, the auction house said.

One of the world's rarest diamonds, about the size of an egg or a large lollipop, is going up for auction next month and is expected to fetch $12 million to $30 million.