Abstract: A lightweight, prefabricated, modular masonry fireplace. The firebox is pre-cast in modular sections of lightweight vermiculate concrete. A Rumford-style version of the fireplace is assembled from lightweight modular sections. The firebox is cast in modular sections. A central body section of the firebox is placed upon a base, and smoke plenum sections are placed upon the central body section. The central body section, and in some embodiments a portion of the smoke plenum section, define and mostly surround the combustion chamber. The central body section includes a plurality of pre-cast planar sections interconnected to collectively constitute the central body portion. All modular components are fashioned from lightweight vermiculite concrete; the combustion chamber is lined with a layer of refractory cement molded to appear as brick, and cast integrally with the sections surrounding the combustion chamber.