Rocks are made out of crystals of different kinds of minerals, or broken pieces from crystals. Some of them are made of once-living creatures or plants. If you go on one or both of these links- http://geomaps.wr.usgs.gov/parks/rxmin/rock.html
http://www.epd86.org/epweb/LOs/Rocks/what_are_rock_made_from.html. You will find more information.
http://www.epd86.org/epweb/LOs/Rocks/what_are_rock_made_from.html. You will find more information.