Although not necessary for pizza making, this large block of stone, ceramic, or synthetic material assures a crisp crust every time. A flat stone gets very hot, sort of like a griddle.
It thus replicates a professional pizza oven in your home: the dough lies right on the heated surface of the stone the same way it sits on the floor of a pizza oven. If you don't use a stone, form the crust on a large baking sheet.
As a general rule, buy the largest stone that will fit comfortably on your grill rack and/or in your oven. It's almost impossible to make a 13 inch pizza fit on a 13 inch stone without the pizza it self slipping and dripping off the edge.
The stone must be preheated for 30 to 45 minutes to ensure that it's hot enough to cook the crust from the bottom up. If a room temperature stone is placed in a hot oven or on a grill, that stone may crack. To avoid this catastrophe, place the stone on the unheated section of the grill rack or on the oven rack positioned in the middle of the oven the moment you begin preheating either. Never place the stone directly over the heat source on the grill. Grill heat is much hotter than oven heat; direct heat can fracture a stone. A pizza stone will get browned and stained over time.
Since it's porous, it will absorb almost any food that gets on it melted cheese, olive oil, burned bits of flour, pepperoni grease. That said, never clean it with soap or other cleaning products.
These will get into the pores, only to come back out and onto the crust the next time the stone is heated. Instead, scrape off any browned material while the stone is still hot, then cool the stone before you wipe the stone with a damp paper towel. The stone will sanitize over the intense heat the next time it's used.
If you don't want to go to the expense of buying a pizza stone, you can buy inexpensive, unglazed ceramic tiles at most home remodeling stores or almost all tile stores. Look for flat but thick tiles that fit tightly together uneven seams can make it difficult to get the pizza on and off them with the peel. Set the tiles tightly against each other on the unheated grill or oven rack, but do not line them up all the way to the sides of your oven. Air must circulate, leave at least two inches of open space around the tiles so heat can rise and move freely in the oven.