A viridium tracker was an isotope that, if chewed and absorbed into the bloodstream by a person, allowed them to be tracked over long distances. It could be destabilized and rendered ineffective by an injection of noranium hydride.
In 2399, Commodore Oh gave Doctor Agnes Jurati a viridium tracker after showing her the Admonition. Narek used the tracker to follow La Sirena after it left the Artifact, until Jurati neutralized it with noranium hydride. (PIC: "Nepenthe", "Broken Pieces")