It's not a rock, it's an improvised ballistic device.
http://www.somosunpueblo.com/March%2030/English.htm
100,000 pro-illegal alien supporters to shut down the city of Chicago, in Protest of the Illinois Minuteman Project and in protest of "THE RACIST" proposal HR 4437
" The city will be paralyzed (with the march) and our voice is going to go up to Washington ", mentioned Radio Host El Pistolero "The Gunman."
Que Bueno 105.1FM
http://www.soundclick.com/bands/pageartist.cfm?bandID=267709