MANY marine organisms etch calcareous substrata(1). Indeed sponges, mainly of the genus Cliona, are important factors in the erosion of calcareous coasts(2,3). Among terrestrial organisms, only a few lichens are known to penetrate siliceous rocks(4,5), an ability unknown in the animal kingdom. The Demospongiae have a siliceous skeleton formed by spicules(6) of various shapes and sizes, but several species also incorporate sand grains or foreign spicules(7,8). The demosponge Chrondrosia reniformis Nardo has no autochtonous spicules but incorporates a nide range of foreign materials in its ectosome(9,10). Here we report that quartz particles are strongly etched and made uniform in size, quickly and with sharp selectivity, the hydrated silica (chalcedony and opal) remaining unaltered. The presence of a thick collagenous ectosome(11) suggests that ascorbic acid, the reducing agent in proline hydroxylation, might be involved in quartz etching by C. reniformis.