Relics are divided into three categories: First-class relics are part of the saint's body, second-class relics are clothing or other articles used by the saint during his life, and third-class relics are any other object that has been touched to a first-class relic.
Obviously, first-class and second-class relics are rare, but there can indeed be an unlimited number of third-class relics.
Relics - from the Latin "reliquiae," meaning remains - have been venerated since the early days of the Church. Just as we often keep a special possession belonging to a deceased love one to remind us of that person, so the early Christians preserved the bodies and possessions of the saints and martyrs. The first non-scriptural reference to relics is found in a 156 A.D. account of the martyrdom of St. Polycarp. By the tenth century it had become common to house relics in churches or shrines, a practice that continues today.
From Pastoral Answers to Questions about the Faith
by Rev. John Patrick Bertolucci,
published by Our Sunday Visitor, 200 Noll Plaza, Huntington, IN 46750