The diamond ring which was bought for \u00a310 has been sold at a Sotheby\u2019s auction for \u00a3656,750.\r\n\r\nThe jewel was purchased 30 years ago at a car boot sale at West Middlesex Hospital in Isleworth, west London. The owner wore the ring for years unaware that it was a 26-carat diamond.\r\n\r\nThe owner believed it was a piece of costume jewellery due to its \u201cfilthy mount.\u201d\r\n\r\nIt was only when the owner, who does not want to be identified, took it to Sotheby\u2019s that its true worth was realised. The auction house originally valued the ring at \u00a3350,000.\r\n\r\nJessica Wyndham, head of Sotheby\u2019s jewellery department, said: "The owner would wear it out shopping, wear it day-to-day. It's a good looking ring.\r\n\r\n"No-one had any idea it had any intrinsic value at all. They enjoyed it all this time."\r\n\r\nAnother highlight of the sale was a Cartier diamond brooch worn by Margaret Thatcher on a number of high-profile public occasions, including the day she offered her resignation as Prime Minister to The Queen.\r\n\r\nThe brooch sold for\u00a0\u00a381,250, exceeding its \u00a325,000-35,000 estimate. The proceeds of that sale will be donated to the Endeavour fund - a charity that supports the recovery of wounded, injured and sick servicemen and women.\r\n\r\nOverall, the auction made sales amounting to \u00a35.2m.