French Truffles are the fruiting body of an underground mushroom; There are hundreds of species of truffles, but the most  fruiting truffles are very expensive as a food.  One of the things that has made truffles so highly expensive is the fact that  truffles are very difficult to cultivate. The culinary world increase their usage of truffles as they  became more widely known and more available. Truffles are  still very much a specialty item and very expensive one at that