The Stone of Gol was an ancient Vulcan psionic resonator that could kill by amplifying violent thoughts, reassembled by mercenaries in 2370.
Vulcan legend described a weapon from the time before Surak that turned its wielder's aggression into a lethal force. Most Vulcans considered it a myth, but fragments survived, and a band of mercenaries led by Arctus Baran set out to collect them.
Picard infiltrated the group under an assumed identity, and the crew discovered that the Vulcan Tallera was using the mercenaries to recover the weapon for an isolationist movement. When she tried to use it on the Enterprise-D, Picard realized that it could only harm those with violent thoughts, and the crew's calm left the stone powerless.