Its owner had not realised it was in fact a 19th century diamond ring until earlier this year when she was encouraged to have it valued.
Bought at a car boot sale at West Middlesex Hospital in west London, the claw-set 26.27ct cushion-shaped diamond ring was taken to Sotheby’s and given an estimate of £250,000-350,000 earlier this year.
Named the Tenner, the diamond has a colour grade of I and impressive clarity grade of VVS2.
Also offered as part of Sotheby’s Fine Jewels sale on June 7 in London, a Cartier diamond brooch worn by Margaret Thatcher was valued at £25,000-35,000 and sold at a hammer price of £65,000.
Thatcher wore it on a number of high-profile public occasions, including the day she offered her resignation as prime minister to the queen. The proceeds of the sale of the brooch will be donated to the Endeavour fund - a charity that supports the recovery of wounded, injured and sick servicemen and women.
Sotheby’s Fine Jewels auction achieved a hammer sales total of £4.15m, above its estimate of £2.9m-4.18m, with 80.1% of the lots sold.
Jessica Wyndham, Sotheby’s head of London jewellery department, said: “It was a thrill to bring the hammer down on two objects which have been the subject of so much interest and attention over the last few weeks."