The church of St John the Baptist, known as the Duomo, was erected by Queen Theodolinda in 595. It was rebuilt in the 13th century, restored in 1577, and repainted in 1739; the façade is the work of Matteo da Campione and his pupils (1369–98). In the chapel where Theodolinda and Agilulf are buried, the Iron Crown is preserved, so called because, according to tradition, it was fashioned from a nail of the Cross of Our Lord, discovered by St Helena. In the high chapel, the antependium in embossed silver-gilt plaques with gems and enamels was executed by the Milanese goldsmith Borgino del Pozzo between 1350 and 1357; it depicts the Baptism of Christ, the symbols of the Evangelists, and episodes from the life of St John the Baptist.
The treasure, preserved in an adjoining octagonal room, was first removed in 1324 and taken to the papal court at Avignon; it returned in 1344. The French sent it to Paris in 1799, whence it was restored in 1815. Among its holdings are: the so-called “embroidery of the Madonna,” which is in fact an Eastern textile of the 6th or 7th century; the so-called “veil of the Madonna” in Eastern silk, dating to the 11th century; a gilded silver dish depicting a hen with seven chicks, attributed by some to the time of Theodolinda and by others to the 12th or 13th century; two parchment fragments of a letter by St Gregory the Great (590–604); the votive cross of Agilulf in gold and gems, part of his crown, stolen in Paris in 1805; the cross known as Berengar’s, belonging to Theodolinda’s crown; a book-cover for an evangelistary, another for a sacramentary in ivory with openwork and silver plaques (9th century). More important is the “gold binding of Theodolinda,” as attested by the inscription thereon; a diptych of gold plaques set with precious stones, pearls, and cameos, the work of a cultured Byzantine goldsmith. Some identify it with the “Persian theca” sent by Gregory the Great to Theodolinda for the baptism of her son Adaloald, while others regard it as an encolpion. A processional cross of gold and silver plaques with scenes from the lives of Christ and the Baptist, featuring in the centre a medallion of the Madonna and Child with Theodolinda kneeling; it was donated by her and remade in 1487 and 1615.
There are also three ivory diptychs: the first is consular, but the two figures were later altered, one into King David and the other into St Gregory; the second seems to depict Stilicho with his wife Serena and their son (late 4th century); the third, known as “of the poet and the muse,” is from the late 4th century. K. Weitzmann and S. Schultz propose to identify the poet as Claudian (Zur Bestimmung des Dichters auf dem Musen-diptychon von M., in Jahrbuch des deutschen archäologischen Instituts, 49 [1934], pp. 128–38). Also preserved are two collections of ampullae (v.), one consisting of 28 glass phials containing oils taken from the shrines of Roman martyrs and brought between 590 and 604 by a certain John to Queen Theodolinda, each with descriptive labels and a summary note (v. INDEX OLEORUM); the second group comprises 16 metal ampullae (tin-lead alloy) containing oils or earth from the shrines of Palestine; they are adorned with depictions of these shrines (late 7th century). The Chapter Library is highly valuable for its manuscripts and incunabula.