If you're intrigued by Naples' cultish love affair with San Gennaro, eye up his glittering treasury at the Museo del Tesoro di San Gennaro, adjacent to the Duomo. Gifts made to Naples' patron saint include ambitious bronze busts, silver ampullae and even a gilded 18th-century sedan chair used to transport his bust on rainy procession days. The star attraction, however, is Matteo Treglia's extraordinary 18th-century mitre, adorned with 3694 gems: 3328 diamonds, 198 emeralds and 168 rubies.
Upstairs, the Sacrestia dell'Immacolata (Sacristy of the Immaculate Conception) shines with 17th-century frescoes by Luca Giordano and Giacomo Farelli. The frescoes in the adjoining Antesacrestia are the work of Francesco Maria Russo, their solid hues reminiscent of his more famous work inside Naples' Cappella Sansevero. Luca Giordano makes another appearance in the adjoining Sacrestia Nuova (New Sacristy), his signed ceiling fresco one of the few works the artist began and completed alone.