Located in the piazza San Francesco, the basilica of San Francesco is among the most important churches in Siena. Built in the 13th century in the Romanesque style, it was enlarged in the following two centuries into the current impressive gothic structure. The façade is brick and has a rather sober style. The only decorations, in marble, are linked to the symbols of the four evangelists and the coats of arms representing the city of Siena and the families who contributed to the maintenance and restoration of the church. The rose window is the only ancient element, acquired from the 15th century façade by Francesco di Giorgio Martini. The building is constructed according to the design of the Egyptian cross, with a single large trussed nave. Inside the church, there is a pyx with the Sacred Particles. The history of the Miracle dates back to August 14 1730 when the pyx containing the 351 pieces of the sacrament was stolen. Three days later, on August 17, the articles were rediscovered by an altar boy in the alms box in the environment of the Collegiate of Santa Maria in Provenzano; the motive behind the theft and especially behind the returning of the sacramental bread by the thief is still unknown. For hygiene reasons they decided not to consume the pieces of the Sacremental Bread, which were carried the next day in a great procession to San Francesco. The faithful chose to preserve the Particles in order to worship them for reparative ends. The pieces of the host were first put in a corporal and then after the canonical visit of the Padre Generale, into a new sealed pyx. After more than 280 years the particles are still intact, in spite of their natural tendency towards deterioration.

Source: wikipedia.org