Under hydrothermal conditions (3 kb, 100° to 300°C, 25 to 5,200 hr), freshly solidified silica gel crystallized to quartz, predominantly in the form of chalcedonic spherulites. No intermediate crystalline phases in the transition from amorphous silica to quartz were detected. Chalcedony that made up the synthetic spherulites was either length-slow, length-fast, or both, depending on the experimental conditions. These findings contrast with the hypothesis that chalcedony derived from silica gel should be length-fast. In experiments in which blue-green algae had been embedded in the silica gel, spherulite nucleation occurred preferentially on surfaces of paniculate organic matter, and the chalcedony generally was length-slow. A summary of spherulitic crystallization mechanisms is presented, and emphasis is placed on impurity and viscosity requirements. Considering these requirements and the experimental results, a viscous colloidal origin is postulated for cherts and other siliceous sedimentary rocks that contain abundant silica microspheres. The seemingly biological attributes of quartz microspheres in Precambrian iron formation cherts can be explained by strictly inorganic processes related to spherulitic crystallization of colloidal precursors.