GENEVA, SWITZERLAND (REUTERS) - The Unique Pink, a 15.38 carat pink pear-shaped diamond is the largest and rarest shade of fancy vivid pink to ever be put on the market and could fetch between US$28 million (S$38.3 million) and US$38 million when it sells at an auction.

"This extraordinary diamond is one of the rarest diamonds in the world. It's been graded by the Gemological Institute of America (GIA) as 'fancy vivid pink', the rarest of all the pink gradings possible. And at 15.38 carats it's also the largest fancy vivid pink pear-shaped stone ever to appear on the market," said Sotheby's International Jewellery Worldwide Division Chairman David Bennett.

The stone falls into the type 2A sub-category, which applies to less than 2 per cent of stones and refers to the purest ones, making this pink pear-shaped diamond all the more remarkable and unique.

"Most diamonds have an impurity inside the stone, commonly it's nitrogen, but type 2a (diamonds) are free of any impurity. So they're extraordinary stones," Bennett added.

The diamond will be auctioned at Sotheby's in Geneva on May 17.