San Gennaro is the cathedral of Naples, named after the martyr-bishop of the same city who died around 305 during the persecution under Diocletian. In the English-speaking world he is known as Januarius, I believe.
The saint is very famous in Italy because of the miracle of the liquefaction of his blood, first reported in 1389. The dried blood is safely stored in small capsules in a reliquary. When these capsules are brought into the vicinity of his body, the dried blood liquefies.
Thousands of people assemble to witness this event in the cathedral of Naples. The archbishop, at the high altar amid prayers and invocations, holds up a glass phial that is said to contain the dried blood of the city’s patron saint. When the liquefaction has taken place, the archbishop holds up the phial again and demonstrates that liquefaction has taken place. The announcement of the liquefaction is greeted with a 21-gun salute at the 13th-century Castel Nuovo.
Anyways, here are some nice exhibition pieces from the treasury of the cathedral: