A diamond ring bought for £10 at a car-boot sale 30 years ago is expected to fetch £350,000 at auction. The owner believed the "exceptionally sized" stone was a piece of costume jewellery when she bought it at West Middlesex Hospital in Isleworth, west London, in the 1980s. Unaware it was a 26 carat, cushion-shaped white diamond from the 19th Century, she wore it daily for decades.