A 26-carat white diamond dating from the nineteenth century. This was what one woman had at home without knowing it. Until the day she decided to have an expert look at this 'unsurprising' and 'dirty' ring she had bought for £10 in the 80s at a boot sale in London.At the time, the exceptional size of the stone had intrigued her but she never could have imagined that it was a real diamond - wearing it as a piece of costume jewellery all this time. More than thirty years later, the Gemological Institute of America has delivered its verdict: this jewel is very expensive. On June 7th, 2017 it was sold at auction at Sotheby's - for £657,000.Jessica Wyndham, director of the jewellery section of the auction house said: 'This is an unprecedented bargain, an incredible find.' It certainly is... now we're off to look up when the next boot sale near us is!