A diamond discovered at a British car boot sale in the 1980s sold for £656,750 ($851,357) at auction yesterday.

The consignor was presumably very happy with that result, given that they paid just £10 ($15) for it.

The diamond was cut in the 19th century 

They wore it daily, assuming it was just costume jewellery and only brought it in to Sotheby’s to be valued on a whim.

Jessica Wyndham, head of jewellery at Sotheby’s, explained: "The owner would wear it out shopping, wear it day-to-day. It's a good looking ring.

"But it was bought as a costume jewel. No-one had any idea it had any intrinsic value at all. They enjoyed it all this time.

"They'd been to quite a few car-boot sales over the years. But they don't have any history of collecting antiques and they don't have any history of collecting diamonds.

“This is a one-off windfall, an amazing find…”

The ring was overlooked partly because the 19th century mounting was dulled by age, but also because of the stone’s unfashionable cut.

Wyndham explained: "With an old style of cutting, an antique cushion shape, the light doesn't reflect back as much as it would from a modern stone cutting.

“Cutters worked more with the natural shape of the crystal, to conserve as much weight rather than make it as brilliant as possible."

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