Verilium was a rare, naturally-occurring mineral compound.
History and specifics[]
Verillium could be found on the planet Mevinna, along with other rare substances such as obsitrate and pyretimite. When mixed, particles of verilium and obsitrate polarized to block sensor scans.
In the year 2268, members of a crashed shuttlecraft crew hid the Excalibur weapon prototype from renegade Andorians by submerging it in a deep pool of water containing verilium and obsitrate. Andorians caught up with the Starfleet officers, but could not find the weapon in the lake and resorted to having to spy on the location in hopes of locating it. (TOS - The New Voyages short story: "The Enchanted Pool")