Small rounded black stones with engraved characters are a kind of Ancient long-range communication device that allows people to see events over great distances through a sort of psychic connection. One such device had been among the artifacts discovered on P3R-233 by SG-1 and brought back to Earth. Another had been found in an archaeological dig in Egypt and was sold years later by the owner's grandson at a garage sale in Indiana. The stone devices need to be initialized by someone who possesses the Ancient gene, and SG-1 theorized that O'Neill, who possesses the gene, must have activated the link between the two stones when he handled the one that was being catalogued and stored in the base archives at the SGC. Joe Spencer, the barber in Indiana who purchased the stone at the garage sale, also possesses the gene, and all that was required was a certain proximity to the stone for a connection between the two to be activated. For seven years, Joe was able to see visions of SG-1 missions as they were being reported in real time in O'Neill's mission reports, while O'Neill witnessed elements of the life of a barber in Indiana.
Although it was originally believed that the stones could only be used by someone with the Ancient gene, it was later determined that the stones, like a lot of Ancient technology, need only be initialized by someone with the gene, as O'Neill had done, after which they could be used by anyone. The stones had been set for O'Neill and Joe Spencer, however, after the Spencer incident, Dr. Lee was able to reset the stones, and both remained at the SGC.
Among the artifacts discovered with the Ancient treasure at Glastonbury was a terminal of Ancient design, and it was determined that the stones were meant to be used in conjunction with the base terminal. Daniel and Vala placed the stones in the proper slots in the terminal and immediately passed out as their consciousnesses reached beyond the Milky Way allowing them to communicate with inhabitants of the Ori galaxy through the bodies of Harrid and Sallis Cicera, whose own consciousnesses remained suppressed. Once the device was activated, however, SG-1 was unable to remove the stones from the terminal or break the psychic connection. A similar set of stones and a terminal discovered in Ver Ager allowed Daniel and Vala to briefly open another connection back to their own bodies on Earth, however, it was only when the terminal and stones at the SGC were thrown into a developing kawoosh that the devices were vaporized and the connection was severed.
Because the stones and the base terminal have been linked to the technology of the Alterans, predating the Ancients' arrival in this galaxy, they have been described both as Ancient and as Alteran communication stones. Although the stones on Earth were destroyed, SG-1 had been working to artificially emulate the technology. Using a Tok'ra subspace communicator and the data gathered while the devices were being used by Daniel and Vala, Carter had tried to mimic the frequencies sent and received by the stones. At the same time, the terminal and set of stones from the Ori galaxy had been removed to Celestis and recovered by the anti-Ori underground movement. The devices allowed Vala, in the Ori galaxy, to open a psychic link and communicate with Earth through the body of Daniel, most likely because Carter's data had been taken from the stone activated by Daniel. It was expected that the connection could be severed by shutting down the computer, however it was instead unexpectedly terminated when Tomin discovered the plot and used an Ori staff weapon to destroy the devices in Ver Isca.
Cross Reference: Ancients, Ancient Communication Terminal, Ancient Gene, Harrid Cicera, Sallis Cicera, Egypt, Garage Seller, Daniel Jackson, Vala Mal Doran, Jack O'Neill, P3R-233, Joe Spencer, Ver Ager
Episode Reference: Citizen Joe, Avalon, Origin, Crusade