The earliest mention of the "Ring of the Fisherman" worn by the popes is in a letter of Clement IV written in 1265 to his nephew, Peter Grossi. The writer states that popes were then accustomed to seal their private letters with "the seal of the Fisherman", whereas public documents, he adds, were distinguished by the leaden "bulls" attached. From the fifteenth century, however, the Fisherman's ring has been used to seal the class of papal official documents known as Briefs. The Fisherman's ring is placed, by the cardinal camerlengo on the finger of a newly elected pope. It is made of gold, with a representation of St. Peter in a boat, fishing, and the name of the reigning pope around it.
"The Ring of the Fisherman." The Catholic Encyclopedia. Vol. 13. New York: Robert Appleton Company, 1912.