The chunk of ambergris - a waxy rock-like substance that can prolong the scent of perfume - was found washed-up on the shore at Anglesey, north Wales, earlier this year.
The ambergris, that measures 8 inches by 6 inches and weighs 1.1 kg, could have been floating in the sea for decades before it came ashore.
Ambergris is produced in the intestines of sperm whales to protect them from the sharp beaks of squid which they eat and is passed as vomit.
"Our vendor was out walking his dog when it ran over to this object and wouldn't leave it alone," said Chris Surfleet, senior valuer and auctioneer.
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"He took it home, did a bit of research and suspected it was ambergris and knew it had a value so came to us. We are 100 per cent certain it is ambergris.
"A number of people with experience with ambergris have studied it who have confirmed it," he said.
"It is a bit like gold in that it comes down to the price per gram. There is a lot of interest in unusual items these days," he said.