You smell them before you see them. The thick perfume is sweet and earthy. In Richerenches in Provence, small clusters of men gather round the boots of parked cars speaking sotto voce . It's hard to catch sight of the wads of €100 notes changing hands, harder still to glimpse the treasure we seek. But hidden in those car boots lie sacks filled with the dusty, potato-like lumps of Tuber melanosporum: the precious black truffle or diamant noir .

The chef Michel Rostang wanders through the crowd. His Michelin-starred Paris restaurant is renowned for its truffle menu (only on offer when the truffles are at their best, which is rarely before mid-December and sometimes only in January). "Our clients start ringing in October to be the first to reserve tables." He'll make the 700km trip from Paris once a fortnight in peak season.