LONDON (Reuters) - A rare, pear-shaped, vivid pink diamond will go under the hammer in Geneva next month, where auctioneers expect it to fetch some US$38 million.

The "Unique Pink", described by Sotheby's as "supremely rare and exceptional", weighs 15.38 carats.

"The Gemological Institute of America, the highest body for grading diamonds, have given this stone their highest possible grading, which is vivid pink," Mr David Bennett, worldwide chairman of Sotheby's international jewellery division, told Reuters.

"It's so strong the colour. That makes it a very rare stone indeed, in fact it's the largest pear-shaped fancy vivid pink diamond ever to be offered at auction."

The discovery of gem-quality pink diamonds is rare, according to Sotheby's, which will auction this one as part of its "Magnificent Jewels and Noble Jewels" sale in Geneva on May 17.

"We're estimating between US$28 million to US$38 million," Mr Bennett said of pre-sale estimates during a preview of the diamond in London. "There have been a couple ... of stones that have sold for around US$2 million per carat or a little more historically so it seems to be perfectly correctly estimated ... We will see."

Earlier this week, a 10.10 carat oval, internally flawless, fancy vivid blue diamond sold for US$31.8 million at an auction held by Sotheby's in Hong Kong.