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By Stephanie Nebehay

GENEVA, May 16 (Reuters Life!) - A pear-shaped blue diamond has sold for 5.2 million Swiss francs ($4.93 million), setting a new world record price per carat for any gemstone, Sotheby's said on Friday.

The buyer of the 3.73 carat blue stone was British jeweller Laurence Graff, who also picked up a light pink diamond weighing 6.26 carats for 1.67 million francs, the auction house said.

In all, 423 of 508 lots on offer found new owners at Sotheby's three jewellery sales held in Geneva on Thursday night, netting a total 60 million francs, a statement said.

"We've had great success with important stones, with period pieces and particularly with jewels of noble provenance," said David Bennett, chairman of Sotheby's international jewellery department for Europe and the Middle East.

The star lot, the pear-shaped blue diamond graded "fancy vivid", the most intense colour, weighs just 3.73 carats and is mounted in platinum.

The price per carat was nearly $1.33 million, which exceeded the $1.32 million paid per carat for a 6.04 carat blue diamond it sold last October in Hong Kong, according to the auction house.

Some 62 jewels from the gem box of Lily Marinho, widow of Brazilian media mogul Roberto Marinho and Horacio de Carvalho, fetched 11.6 million francs, nearly doubling their pre-sale estimate, Sotheby's said.

A pair of her ear clips, each dripping with a pear-shaped diamond weighing more than 11 carats, brought 3.07 million francs. The evening's second most expensive lot, it went to an anonymous buyer.

"Lily likes large pieces -- it's Brazil," Bennett told reporters at a pre-sale briefing, recalling his visit to "Brazil's First Lady" at her Rio estate where flamingos roam.

Now 87, the former Miss Paris assembled her collection during a "fairly tale" life, Sotheby's said. Her 1946 portrait by Dutch artist Kees Van Dongen fetched $685,000 at its Impressionist and Modern Art sale in New York last week.

"He was known among society women for increasing the size of the jewels that they wore in his portraits and, as a result, he was very popular with his sitters," Lily Marinho said. (Editing by Paul Casciato)