Saint Helen had one of the nails which pierced the holy Body of Christ in the crucifixion made into a bridle for her son, Constantine the Great.
"A bridle?!" You might ask. "What business does Constantine's horse have chomping on the nail of Christ?
Apparently it was an attempt to fulfill the prophecy of Zechariah 14:20
"In that day that which is upon the bridle of the horse shall be holy to the Lord."
Theodoret in his Ecclesiastical History (ch. 7) records the emperor's sacred bridle and specifically cites it as fulfilling Zechariah 14:20. Pretty cool.
The church historian Socrates (5th century) relates that one of the sacred nails was made into a helmet for Constantine and the other into a bridle.