The black winter truffle (Tuber Melanosporum), also called the "Périgord" truffle (not a trade name but only a qualification of a kind), is one of the most prestigious truffles:

More or less of a round shape, the truffle can go up to the size of an apple and even bigger, but usually weights around 2-3oz. Its skin has a pyramidal scale and its flesh goes from grayish to black with thin white veins when mature. A good truffle should be firm and have a black flesh with thin white veins.

The largest production of truffles comes from the South East of France: in Provence. The biggest truffle market takes place in a small village called Richerenches, in Northern Vaucluse: more than a ton of truffles can be traded there in one day! Truffles can also be found in Spain and Italy.
The season starts in December till March.