Asgard stones are devices which the Asgard use to interact with their technology. Functioning in a manner similar to control crystals, they contain information which must be placed in a specific location on a console. When that happens, the technology will activate. Some stones also function specifically as communication devices. (SG1: "Fair Game", "Enemies", "Red Sky", "Revelations", "Fragile Balance")
Asgard stones are used by not only the Asgard, but the Tau'ri as well, although this is only when using Asgard devices. Samantha Carter activated a time-dilation field around the Odyssey by using the Asgard stones on a console. It is shown that the technology is activated by pressing down on the Asgard stone. Also, Asgard stones aboard the Odyssey seem to be powered by the Asgard power core. These stones have also been seen aboard the Phoenix and Daedalus, but are likely aboard all other Daedalus-class ships as well. (SG1: "Unending") (SGA: "The Last Man", "Search and Rescue")
A stone being used.