This clue was located inside the diamond that Jean-Baptiste Tavernier, a Tomas agent, brought back from India. During the French Revolution, it was stolen by the Ekats and they cut it into smaller pieces. The piece that held the clue was kept in a vault in the home of two high ranking Ekaterina agents, Vladimir and Vera Chernov.
Zinc is a metal. It is needed within the human body for proper functioning.