A rock or stone or, in some cases, a specific stone or rock, is often a symbol with both spiritual and temporal meanings.  In general, a rock or stone represents durability, permanence, strength and stability.  Rocks and stones are long lasting so can also represent immortality.  The "living rock" is a symbol of man's primordial self.  In Christianity, a rock can represent Christ or St. Peter.  Stone can represent the indestructibility of that which is permanent, i.e. the spiritual.  A stone or rock can represent the cosmos or the universe.  Large upright stones such as those at Stonehenge can represent the support of the universe or the center of being that allows man to find paradise or gain enlightenment.  Round stones can represent the moon and female forces or principles.  In many ancient temples, a baetylic stone represented the presence of a deity or of the spiritual sun.  In many nomadic and aboriginal tribes, stones are considered the bones of Mother Earth.  In some part of Africa, a blue stone represents the sky god.  The Philosophers' Stone, the supreme quest of some alchemists, represents the awakened, enlightened self; spiritual, mental and moral wholeness.  The concept of being "blackballed " originally was done by dropping a white or black pebble into a container.  At the ancient Greek temple of Delphi rested a stone known as "the navel of the world" and it represented the center of the world and the cosmos.  One giant rock was considered by the Hopi and other Native Americans to be the heart of Mother Earth and they would hold religious cerimonies near it. 
    One thing about stones and rocks that fascinates me is why did ancient people in many parts of the world build huge temples and building using giant stones weighing many tons each?  There has been a lot of speculation about how this was done, but I've never heard any discussion about why.