Megaquartz, typically as individual crystals showing a number of euhedral faces, inclusion-rich or inclusion-poor, is disseminated throughout the interior of a large specimen of the stromatoporoid Hermatoporoidea from the Middle Devonian Burdekin Formation, northeastern Australia. It represents a replacing, late diagenetic phase occurring after the occlusion of pore space within the skeleton by carbonate spar and was contemporary with, and concentrated near, microscopic pressure-solution seams. Skeletal tissue rather than carbonate spar sponsored megaquartz growth that occurred while an intimate contact between growing quartz crystal faces and the carbonate it replaced was maintained. Spherulitic quartzine, the common type of silicification in limestone of the Burdekin Formation, replaced the exterior surfaces of carbonate skeletal debris in a similar manner. Diffusion on intergranular films is considered to be the mechanism by which both megaquartz and quartzine were emplaced, and the two modes are considered to reflect different replacement rates. Rapid replacement, associated with short diffusional pathways, is attributed to quartzine, whereas megaquartz is thought to represent slow replacement associated with long, diffusional pathways.