A piece of the Stone of Gol.
The Stone of Gol was a psionic resonator used on ancient Vulcan as a weapon which amplified telepathic energy, turning a persons violent thoughts and emotions against them. The weapon became useless among Vulcans, when emotions were abandoned during the Time of Awakening, led by Surak. Following the civil war on Vulcan, the Stone was said to be destroyed by the Vulcan Gods.
The Vulcan Isolationist Movement stole the first fragment in 2369 and hired Arctus Baran to find the two remaining pieces. (TNG: "Gambit, Part II")