Lead

Lead was a metallic element.

Vardans could not broadcast themselves through lead. The Fourth Doctor had the President's office decorated with lead lining to keep the Vardans from reading his mind. (TV: The Invasion of Time)

Professor Sondergaard had a lead-lined laboratory in the Solonian mines. The lead was necessary to protect himself from the high radiation levels in parts of the mines. (TV: The Mutants)

After taking it from the altar set up Maximillian Stael's coven, the Fourth Doctor placed the skull Eustace in a lead-lined box given to him by Adam Colby. (TV: Image of the Fendahl)

On Atrios, areas with high radiation levels such as K-Block were shielded with lead. (TV: The Armageddon Factor)

Torchwood Three kept a radiation sponge in a lead-lined case. (PROSE: Another Life)

On Venus, the Daleks forced Jeff Stone to work at a lead mine. They needed lead to proof their casings against rust in the planet's humid atmosphere. (PROSE: Red for Danger)

On Wengrol, Yend scientists built massive lead buildings to protect themselves from Mortain's radiation while they worked to find a cure for the mutations plaguing their world. (PROSE: The Sons of the Crab)

A lead-line coffin was used at the Eleventh Doctor's faked funeral at UNIT's Mount Snowdon base. Sarah Jane Smith and Jo Jones survived the detonation of the base by hiding inside. (TV: Death of the Doctor)

In Blackcastle, the Fourth Doctor used lead to block a signal from the Wrarth Warriors which would have detonated the bomb they planted inside his body. (COMIC: The Star Beast)

Other references
John Hart remarked to the immortal Jack Harkness that he could encase him in lead and throw him into the Mariana Trench and he would still claw his way out and swim back. (AUDIO: The Death of Captain Jack)