According to family tradition, this English locket (ca. 1597) holds a piece of the caul (birth membrane) of John Monson.
A baby born still covered in its caul was thought to be an omen of good luck. Cauls were dried and saved as luck charms and as protection against drowning.
John Monson himself may have been presented with this locket by his parents as a way to carry the luck of his caul with him wherever he went.
Source: Victoria and Albert Museum