A high stiffness substrate for optical elements is described. The substrate includes a graphite finishing layer and a non-oxide ceramic base substrate. The non-oxide ceramic base substrate is preferably a carbide, such as boron carbide or silicon carbide. The graphite finishing layer may include a surface with low finish. Low finish may be achieved by diamond turning the graphite surface. The graphite finishing layer may be joined to the non-oxide base ceramic with a solder. A supplemental finishing layer may be formed on the graphite finishing layer. A reflective stack may be formed on the graphite or supplemental finishing layer. Methods for making the substrate are also described.