LONDON: Precious stones from Britain?s crown jewels were hidden in a biscuit tin and buried at Windsor Castle during World War II, a BBC documentary to be shown on Sunday reveals. Gems, including the Black Prince?s Ruby from the Imperial State Crown, were buried under a secret exit from the mediaeval castle used in times of emergency. The operation, intended to ensure the priceless gems did not fall into Nazi hands, was ordered by Queen Elizabeth II?s father, king George VI. It was such a closely-guarded secret that Queen Elizabeth, 91, who spent the 1939-1945 war at Windsor Castle for safety, did not know the details. ?What was so lovely was that the Queen had no knowledge of it. Telling her seemed strangely odd,? said royal commentator Alastair Bruce, who presents the documentary.