Rodinium

From Memory Alpha, the free Star Trek reference

Rodinium was one of the hardest known materials and it was frequently used in the construction of spacecraft and fixed installations. In 2266 rodinium was considered the hardest material known to Federation science, more than 21.4 times harder than diamond.

The Earth Outpost Stations along the Romulan Neutral Zone had armor made from cast rodinium, although that was insufficient to protect them against the attack of a Romulan Bird-of-Prey's plasma torpedo. (TOS: "Balance of Terror")

Rodinium was also a common material in Cardassian starship construction. The debris analysis of the Bok'Nor found remnants of rodinium. (DS9: "The Maquis, Part I")