Osmium

Osmium is a strong metal, often utilized for its thick shielding properties.

NX Class starships of the 22nd century used osmium as the primary shielding in their warp Nacelles, and the outer shell of the catwalk.

This thick protection helped the crew of Enterprise on two occasions; when they had to pass through a neutronic storm, the entire crew took refuge in the catwalk, and the osmium protected them from the deadly neutronic radiation. (ENT: "The Catwalk")

The crew took refuge there again, when non-corporeal beings tried to take over their bodies. It was found that they couldn't pass through the osmium alloy of the catwalk. (ENT: "The Crossing")

Much of the surface rocks of the Kalandan homeworld was composed of a Diburnium-osmium alloy. (TOS: "That Which Survives")

Mr. Scott's Guide to the Enterprise speculates that a majority of the metals the Enterprise was made of was osmium.