Rodinium
Rodinium is one of the hardest known materials and it is frequently utilized in the construction of spacecraft and fixed installations. In 2266 rodinium was considered the hardest material by Federation, greater 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 is also a common material in Cardassian starship construction. The debris analysis of the Bok'Nor found remnants of rodinium. (DS9: "The Maquis, Part I")