The Eye of the Serpent is a large ruby that was housed in the Tower of the Serpent, a place of worship under influence of the snake cult lead by Thulsa Doom. It was stolen from the sacrificial pit of the tower by Conan and Subotai in their cooperative robbery with Valeria. The Eye of the Serpent was guarded by a large snake that Conan and Subotai killed after accidentally awakening it.
The gem was incorporated into a necklace that was given to Valeria as a gift from Conan.
The Eye of the Serpent was a prized possession of Thulsa's, as he was curious of its whereabouts in the first conversation he had with Conan.