Victurium

From Memory Alpha, the free Star Trek reference.

Victurium is a metal alloy used in the Federation starship superstructures. Large amounts of victurium alloy, such as the amounts used in the construction of Oberth-class vessels, destabilize the transporter systems of most starships. In 2368, the USS Enterprise-D encountered this difficulty when attempting to investigate the damaged SS Vico. (TNG: "Hero Worship")