LONDON, March 20 (Reuters) - A 59.60-carat mixed cut diamond known as "The Pink Star" will be auctioned in Hong Kong next month by Sotheby's which said it could fetch upwards of $60 million to make it the most expensive diamond of its kind ever sold.

The pink diamond is the largest Internally Flawless Fancy Vivid Pink diamond ever graded by the Geological Institute of America (GIA), yet the sparkling stone is still small enough to fit onto a ring, the auction house said.

Sotheby's said that the diamond, which was initially mined by De Beers in 1999 as a 132.5 carat rough diamond before being cut and polished, was the most valuable polished diamond ever offered at auction.

"The extraordinary size of this 59.60-carat diamond, paired with its richness of colour, surpasses any known pink diamond record in history," said David Bennett, worlwide Chairman of Sotheby's Jewellery Division.

The auction will take place in Hong Kong on April 4.

(Reporting by Alistair Smout; editing by Guy Faulconbridge) ((alistair.smout@thomsonreuters.com; +44 207 542 7064; Reuters Messaging: alistair.smout.thomsonreuters.com@reuters.net))