The group of fragments that gives this room its name, in carved marbles and decorated with mosaic, comes from the ancient façade of the Cathedral, begun by Arnolfo di Cambio in 1296. When this architectural front was dismantled in 1587, much of the wall decoration was lost. The few surviving fragments, now on display, were discovered during restoration works carried out after the 1966 flood: they had been reused, upside down, in the sixteenth-century floor of the Cathedral.

Among the fragments are tiles and cornices that marked out the façade, dividing the spaces intended to house the statues. The mosaic-decorated pieces can be referred to the façade of Arnolfo di Cambio: it is likely that Arnolfo employed craftsmen from Rome, a city where he had worked and where the mosaic tradition was firmly rooted. Other fragments, however, belong to a later phase of construction, when, around the mid-fourteenth century, the building site of the Cathedral was directed by Francesco Talenti.

The polychrome marbles, the precious materials, and the use of gold made the façade shine in a way that is difficult to imagine today, especially at sunset, when the western front of the church received the last rays of the sun. These light effects, together with the rich polychromy, gave the Cathedral façade the appearance of a true image of the Heavenly Jerusalem described in the New Testament, whose walls, according to the Book of Revelation (21:18–21), are built of precious stones.