This photo taken in Alderney, one of the Channel Islands, dated June 2012 and released on Friday March 8, 2013 by scientist Guy Ropars shows the Alderney Crystal, a piece of calcite. Researchers say the rough, whitish crystal recovered from the wreckage of 16th century English warship may be a sunstone, a special kind of mineral believed by some to have helped medieval seafarers navigate the high seas. (AP Photo/Guy Ropars)

Associated Press
This photo taken in Alderney, one of the Channel Islands, dated June 2012 and released on Friday March 8, 2013 by scientist Guy Ropars shows the Alderney Crystal, a piece of calcite. Researchers say the rough, whitish crystal recovered from the wreckage of 16th century English warship may be a sunstone, a special kind of mineral believed by some to have helped medieval seafarers navigate the high seas. (AP Photo/Guy Ropars)
This photo taken in Alderney, one of the Channel Islands, dated June 2012 and released on Friday March 8, 2013 by scientist Guy Ropars shows the Alderney Crystal, a piece of calcite. Researchers say the rough, whitish crystal recovered from the wreckage of 16th century English warship may be a sunstone, a special kind of mineral believed by some to have helped medieval seafarers navigate the high seas. (AP Photo/Guy Ropars)
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