Once thought to have been created by the striking of lightning on tree trunks, truffles actually develop in symbiosis with the roots of Poplar, Lime and Oak trees. Resembling knotted tubers, there three main varieties: the White Truffle, the Black Truffle and the Black Summer Truffle.

In San Giovanni d'Asso, This year's 'White Truffle of the Crete Senesi' exposition at is being held the 9th and 10th, and 16th and 17th November. San Giovanni d'Asso was the first area in the county of Siena to inaugurate the festival 'un tartufo per la pace' (a truffle for peace) and each year a leading figure is offered the best example truffle as a symbol of world peace. In the past, Clinton and Gorbachov have been offered the year's best truffle. In this way, the truffle has become an ambassador for the territory, and not simply for its gastronomic qualities.

The characteristic colour, taste and fragrance of the truffle are determined by the species of tree with which they grow. Oak tree truffles are darker in colour and have a richer smell while truffles found on the roots of Lime trees are paler and more aromatic. The white truffle is held to be the best variety, the 'King of Truffles'. It is found throughout the foothills of the Appenines, including the Crete Senesi region in the county of Siena. The form of the truffle depends on the type of terrain in which it grows. In loam soils truffles are more smoothly formed. In clay soils the lack of space in which to develop gives the truffle a more gnarled texture