The mystery surrounding the “Gem of Tanzania” ruby deepened yesterday after industry experts said they had never heard of that particular stone, nor of a ruby worth that much.

The stone has appeared on the books of Wrekin Construction, a Shropshire construction company that collapsed last week. Its 2007 accounts reported the stone worth £11m but the books of Tamar, its parent, previously valued the stone at £300,000.

Sean Gilbertson, an executive director of Gemfields, a coloured gemstone expert, said: 'A stone like this would be so exceptionally rare I don’t have the words to describe the chances of its existing. Put it this way: I wouldn’t use my money to buy it.'

Financial Times