A 59.6-carat pink diamond is expected to become the most expensive diamond ever auctioned when it comes up for sale next month.

Sotheby's plans to auction the famous "Pink Star" diamond — an internally flawless, fancy vivid pink stone — at its sale in Hong Kong on April 4.

The auction house estimates it will sell for more than $60 million. If it fetches that price, it will become the most expensive diamond ever sold at auction, topping the $57.5 million sale price of a 14.6-carat blue diamond sold by Christie's last year.

Sotheby's says demand for investment-grade diamonds remains strong — especially for the best, rarest stones.

"The extraordinary size of this 59.60-carat diamond, paired with its richness of color, surpasses any known pink diamond recorded in history," said David Bennett, worldwide chairman of Sotheby's Jewellery division.

Yet the Pink Star has a history to overcome. The stone was sold by Sotheby's in 2013 for $83.2 million. Yet the buyer, diamond cutter Isaac Wolf, defaulted on the payment and the stone was returned to Sotheby's. The company then sold off partial ownership of the stone to two companies — Diacore and Mellen — and continued to hold it as part of Sotheby's inventory.

Sotheby's financial statements have valued the stone at $72 million as part of its inventory.