Within Fortnite, there are three main Materials you can find and gather. These three materials are Wood, Metal, and Stone, and they each help players build structures and Craft other items.
Wood is obtained by breaking down anything made of wood (primarily trees), which you can obtain in abundance in Forest Zones, Suburban Zones, and anywhere with loads of Wooden Buildings.
Metal is obtained by breaking down anything made of metal, making Mines, Suburban Zones, Industrial Zones, and Metal Buildings you best places to finding materials made of such.
Finally, Stone is obtained by breaking down anything made of a given type of stone, which is why Rocks (obviously), Suburban Zones, Industrial Zones and Stone Buildings give you your best shot at obtaining materials made of stone.