A low density substrate material for reflecting optics. The substrate material is a magnesium alloy or composite material that is capable of being finished by diamond turning to form an optically smooth surface with low root-mean-square roughness. The finish quality of the diamond-turned surface is sufficiently good to permit use of the magnesium material as a substrate for a reflecting optic without further processing. The magnesium substrate material contains at least 80 wt % Mg and may also include Al, Si and/or other elements. The density of the magnesium substrate material is much lower than the density of current Al alloy substrate materials.