Diburnium is a metallic element.
An abandoned Kalandan outpost was built on an artificial planet composed primarily of a diburnium-osmium alloy. (TOS: "That Which Survives")
The prison door bars used by the Kelvans to hold Captain Kirk and his landing party in a cave was described by Spock as "similar to diburnium, but much denser". (TOS: "By Any Other Name")