The exquisitely beautiful bas-relief (1414-21), which gives the Porta della Mandorla its name, is the work of the Florentine sculptor Giovanni di Antonio di Banco (c.1380/90-1421), better known as Nanni di Banco.
The door is located on the north side of the Duomo and the almond-shaped motif, in the centre of the relief, depicts the Assumption of the Virgin Mary (Her ascension to Heaven). Mandorla is the Italian word for almond.
The Madonna hands down her girdle to Saint Thomas (the apostle who doubted Christ's resurrection), as proof of her Assumption into Heaven. The actual girdle, which was first made of silk and then of copper, has long since disappeared.
According to extra-biblical tradition, Thomas arrived too late to see the Assumption. The Virgin, therefore, offered her girdle to the incredulous apostle as tangible proof of her departure.
This scene is common in Tuscan art, because what is believed to be the actual relic of the Holy Girdle, known as the Sacra Cintola, is kept in Prato Cathedral.
In addition to Saint Thomas, there is, in the lower-right corner, a curious image of a bear shaking an oak tree. This strange image has always perplexed scholars. One explanation is that in the medieval belief system bear cubs were thought to be born shapeless, their form was given to them by the licking of their mother. This act became a symbol of Christianity, which, it was held, reforms and regenerates non-Christians.
However, this theory doesn't explain the presence of the oak tree!