It was only one of several pieces which have broken off the building this month. Across the city, slabs of marble have begun falling from the beautiful 15th century green and white facade of Santa Maria Novella.
The imposing Renaissance Palazzo Strozzi, built by bankers, is also crumbling. Pieces are falling off the walls of the Bargello, once the town hall, then the prison and now the home of sculptures by Michelangelo and Donatello.
Large cracks have appeared inside the Palazzo Vecchio, Florence's city hall. Chunks fell off the picturesque Ponte Vecchio last spring and conservation authorities are worried that the vibrations from 500 buses and coaches that pass by and under the Uffizi galleries each day may be weakening its structure.
Florence's incomparable palaces and churches are crumbling and no help is in sight. The facades are surrounded by unsightly barriers, festooned with red and white plastic tape to keep the public out of danger while the buildings rot.
'There are several causes,' says Professor Domenico Valentino, the superintendent for cultural property in Florence. 'The buildings are old, but their natural decay has increased exponentially as a result of pollution.'
And, he adds, neglect. For lack of funds, the buildings have never had the routine maintenance they require to combat decay and avoid the need for huge and expensive restoration work when their condition gets serious.
'If you have a cold you take an aspirin,' Professor Valentino says. 'If you neglect it long enough you could come down with pneumonia, then you need antibiotics. What we are faced with now calls for the equivalent of cobalt therapy. Things would never have got to this point if we had been able to do regular maintenance work.'
Outside his splendid frescoed office in the Palazzo Pitti is a colonnaded courtyard. 'Look,' he says, bending down. The pavement is of pietra serena, the soft grey stone of which many Florentine churches and palaces were built. It was loved by architects because it goes so well with white for the interiors of churches.
He flicks with his nail and a patch the size of a credit card flakes off instantly. 'Look at this,' he goes on, turning to a wall of the Palazzo. It is built of the golden pietra forte, the other great stone of Florence, harder, as its name suggests, than the pietra serena but equally vulnerable.
Luca Pitti, a rich, greedy and ambitious 15th-century cloth merchant, had his palace built of huge, rough, rounded blocks that created an awe-inspiring impression of power. But through these blocks run fine veins of calcium which, 500 years later, are being broken down by the elements and pollution, leaving deep fissures, as if the stone had been cut through with a knife.
'A technician who inspected part of the facade last week said that with one blow from a hammer he could have knocked down a block of around 80 kilos,' Professor Valentino says.
Pietra serena and pietra forte were used everywhere in the city because they were quarried in the nearby hillsides (in the case of the Palazzo Pitti, from the gardens behind) and were easy to work. But they are sandstone and thus particularly affected by acid rain.
To avoid accidents, facades such as those of the Pitti and Strozzi palaces need to be inspected and the endangered blocks pinned. Everywhere the buildings need treatment with special products to protect the stone from pollution. Marble facades are much simpler to repair.
But with Italy struggling to pare down a very large state deficit, there is no money. 'Signor Amato (the Prime Minister) has blocked everything,' Professor Valentino says. To restore the facade of Palazzo Pitti alone would cost 100bn lire (about pounds 50m). The Ministry of Cultural Property has 300bn lire for all the treasures of Italy.
The ministry, and with it a large part of Italy's remarkable cultural heritage, has always been a Cinderella, Professor Valentino says. 'Governments have never been able to get it into their heads that this heritage is the oil wells of the state.'
The current minister, Alberto Ronchey, is preparing to raise funds by marketing reproductions and other products based on Italy's art works and encouraging commercial sponsorship of restoration projects. But Professor Valentino doubts whether sponsorship could solve this particular problem. 'It is not like restoring a fresco - first you can't see much, then afterwards there it is in all its glory. You can't really see the decay and afterwards the buildings won't look any different. It is not something that would do much for a firm's image.'
The buildings are not about to collapse, he emphasises. But if nothing is done, gradually the six million tourists who visit Florence every year will find everything 'chiuso, chiuso, chiuso' - closed, closed, closed. 'It will get to the point where we cannot let people in for fear someone will get killed. If anything happens it will be our responsbility, even though our hands are tied.'