St. Thomas the Apostle
St. Thomas was one of the Twelve Apostles of Christ, most famous for his doubt at the Resurrection. The relic is one half of the middle phalange of the "pinkie finger" from the right hand of the saint.
Our Lord invited Thomas to put his finger into the nail marks in His hands... but forced him to thrust his hand into Christ's side. Consequently, this small digit from Thomas' right hand was actually inserted into the side of the risen Christ.