A large diamond ring is expected to fetch £350,000 at auction - 30 years after it was purchased for £10 at a car boot sale.
The 26.27 carat white diamond stone was initially thought to be fake because 19th Century diamonds were not cut to perfection like today's gems.
However after wearing the ring for "day-to-day chores", the owners brought it into Sotheby's after a jeweller told them it could have substantial value.
Jessica Wyndham, head of London's jewellery department at Sotheby's, said the owners do not want to be named but are "incredibly excited, anyone would be in this position, it's a life-changing amount of money."
Although it was believed to be cut in the 19th Century, its history and how it arrived at the car boot sale is unknown.
She added: "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", she said.
The ring was originally purchased at a Sunday sale at the West Middlesex Hospital in Isleworth, west London in the 1980s and will go under the hammer at Sotheby's in July.