A British man has been offered 50,000 euros ($A65,625) for a strange-smelling rock his dog found on a beach, which may be a rare form of whale vomit used in perfumes, the BBC reports.
Ken Wilman was walking his dog Madge in the coastal town of Morecambe in northwest England when she began "poking at a rather large stone" with a waxy texture and yellowish colour.
At first he left it on the beach but later retrieved the object, which he believes is a piece of ambergris, a substance found in the digestive systems of sperm whales.
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Whales sometimes spew up ambergris, which floats on water and has been highly prized for centuries.
It is used in perfume-making for the musky fragrance it acquires as it ages - but newer ambergris is foul-smelling.
"When I picked it up and smelled it I put it back down again and I thought 'urgh'," Wilman told the BBC on Thursday.
"It has a musky smell but the more you smell it the nicer the smell becomes."
He is waiting for tests to confirm his find is ambergris, known as "floating gold", but says he has been offered 50,000 euros for it by a French dealer.
"It's worth so much because of its particular properties," Andrew Kitchener, principal curator of vertebrates at the National Museum of Scotland, told the BBC.
"It's a very important base for perfumes and it's hard to find any artificial substitute for it."
The substance gets a mention in the classic 1851 whaling novel
Moby Dick.
Author Herman Melville wrote: "Who would think, then, that such fine ladies and gentlemen should regale themselves with an essence found in the inglorious bowels of a sick whale! Yet so it is."
