Diburnium

Diburnium is a metallic element with phaser-resistant qualities.

An abandoned Kalandan outpost was built on an artificial planet composed primarily of a diburnium-Osmium alloy. Attempts to cut through the surface with hand phasers had little effect. (TOS: "That Which Survives")

Kelvans used a material similar to diburnium, though much denser, in security applications. Prison door bars and a Kelvan field projector casing were fashioned from the highly phaser-resistant substance. (TOS: "By Any Other Name")