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Apart from the blessing/adoration of Christ or whatever, the other side has St Christopher... the patron saint of travellers. When you see it with an anchor it's a sign that the medallion was intended to keep someone safe on the water. It does look to be brass and, depending on how big it is, it may have been suspended somewhere on a boat or used as a fob rather than worn around the neck. The loop has a substantial groove worn into it from continual movement.