In the 2017 Instruction, significant relics are the body of a saint or blessed, notable parts of it, or the whole of the ashes; they are kept in sealed urns in places that are secure and suited to veneration. Non-significant relics are small fragments of the body and objects that came into direct contact with the person; they are to be kept in sealed cases where possible and honored without superstition or illicit trade. The two vocabularies do not map neatly: a small bone fragment is first-class in popular usage and non-significant in the Instruction. Objects associated with Christ's Passion are often loosely called first-class as well. Whatever the class, the point of a relic is the person, and beyond the person, Christ.
Faith & pilgrimage · YOUR QUESTION, ANSWERED
What are first-, second- and third-class relics?
In Catholic custom, a first-class relic is the body of a saint or blessed, or a part of it, such as a bone fragment or a lock of hair. A second-class relic is an item the saint owned or used, such as clothing or a book. A third-class relic is an object that has been touched to a first- or second-class relic, such as a rosary touched to a saint's tomb. These three classes are traditional terms and do not appear in canon law. The Holy See's 2017 Instruction on relics distinguishes only significant and non-significant relics.

Related questions, answered
Is the three-class system official Church teaching?
No. It is long-standing customary terminology, widely used by shrines, dioceses and Catholic media, but it is not found in the Code of Canon Law or in the Holy See's 2017 Instruction 'Relics in the Church: Authenticity and Preservation', which uses the terms significant and non-significant relics.
Can I make a third-class relic myself?
In the customary sense, yes: a rosary, medal or holy card that you touch to a saint's tomb or to a reliquary is commonly called a third-class relic. There is no formal process for recognizing one and no certificate. It is a devotional keepsake that reminds you to ask the saint's prayers, not an object with power of its own.
Does a first-class relic come with a certificate?
A relic exposed for public veneration should. The 2017 Instruction says relics may not be displayed for veneration without a certificate from the ecclesiastical authority guaranteeing their authenticity; for fragments taken during a cause, the postulator signs it, or otherwise the bishop or his delegate.
This question may be part of your discernment
- Relics & Saints PilgrimageA relics and saints pilgrimage takes you to the tombs and relics of the Church's holy men and women, to honor what God did in them, to ask their prayers, and to learn from them how to follow Christ.
- American Martyr Pilgrimage: Blessed Stanley RotherA pilgrimage to the Guatemalan parish where Blessed Stanley Rother, a priest from Oklahoma, stayed with his people and was killed, to ask for the grace to stay faithful where God has placed you.
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