According to wiki
Roman Catholic tradition holds that the head on display in San Silvestro in Capite in Rome is that of John the Baptist, discovered for the second time, as also maintained by Pope Benedict XVI in August 2012.
What the Pope said:
The pope also spoke about a fragmented piece of St. John's head that is kept as a relic in the Church of Saint Sylvester in Rome.
“In the Roman Martyrology, reference is made to a second finding of the precious relic, transported, on this occasion, in the church of San Silvestro in Rome. These little historical references help us to understand how how and deeply St. John the Baptist is venerated.”
There is also some similar Islamic tradition about this relic mentioned there.
My question is what did Martin Luther think about the 'preserved head' and other equally impressive holy relics?