LONDON: A diamond ring bought for next to nothing in a London junk sale is expected to fetch £350,000 ($455,000, 450,000 euros), Sotheby?s auction house said Monday.

The owner bought the 26-carat, cushion-shaped, white diamond ring for £10 in the 1980s and wore it while doing shopping and chores, thinking it was costume jewellery, Sotheby?s said.

"The owner would wear it out shopping, wear it day-to-day. It?s a good-looking ring," said Jessica Wyndham, head of Sotheby?s London jewellery department.

"No-one had any idea it had any intrinsic value at all.

"The majority of us can?t even begin to dream of owning a diamond that large."

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The diamond is thought to have been cut in the 19th century, when the style was to cut to conserve the weight rather than to make it as sparkly as possible, hence its relatively dull brilliance.

"It could trick people into thinking it?s not a genuine stone," said Wyndham.

She said the owner, who does not want to be named, brought the ring in after a jeweller told them it could be worth something.

She said the owner was "incredibly excited. Anyone would be in this position: it?s a life-changing amount of money.

"This is a one-off windfall, an amazing find."

The ring will be auctioned on July 7.