Treasury Cathedral of Trier: so-called portable altar of Willibrord (Trier, 8th-14th c.). The porphyry altar stone has an inscription referring to Saint Willibrord.
<a href="//commons.wikimedia.org/wiki/User:Tom-L" title="User:Tom-L">Tom Lemmens</a>
CC BY 4.0
<span class="int-own-work" lang="en">Own work</span>