A mask apparatus for x-ray lithography and metrology where the x-ray absorber material is embedded in diamond and then covered with a thermally conductive material to provide requisite thermal conductivity when irradiated with x-rays. The apparatus then includes a hollow holder that is thermally interfaced with the mask and may also include means for external thermal control. The mask apparatus allows for transmission of x-rays from a lithography beam as well as metrology beams of other wavelengths including UV, IR, visible, and others.