A rock would be defined as specks of the earths crust formed over years of pressure and hot temperatures. They are mostly filled with minerals and and other components. A boulder would be defined as a large detached mass of rock. A boulder would fall in-between the largest rock on the mountain and the smallest. So technically a boulder is formed through a giant mass of earth that stays connected as it detaches itself from whatever, a rock would just be small pieces of it.
