That is an "Iron Eye of Fate" talisman. Once you look at it you are cursed with bad fortune. The owner probably accidentally glanced at it while removing it from it's protective leather pouch while he was being robbed, dropped it on the ground as he tripped on a particle of sand and landed face first onto a rock while his coin pouch flew out of his pocket into the robber's hand.
Unfortunately for you, now that you have looked at it, your luck at Metal Detecting and Fishing is right out the window. All you will find detecting from now on is aluminum cans, pull tabs and nails. If you fish please stay away from water bodies and boats.