Abstract: A box beam comprises at least two elongated component blocks of granite, each containing at least one flat surface, the flat surfaces having identical dimensions. Igneous rock material is removed from the flat surface of at least one of the component blocks so as to form at least one open cavity in that component block. The interior cavity has boundary walls of a predetermined, though not necessarily uniform, thickness. The two component blocks are then bonded together along the remaining unremoved portions of the flat surfaces so as to form a box beam.The system described above for manufacturing hollow granite box beams reduces weight while maintaining stiffness comparable to that of the composite ceramic or metal beams along with the inherent stability of granite, e.g., low coefficient of thermal expansion, in a highly efficient manufacturing process.