Molar-tooth structures generally form a network of interconnected vertical and horizontal calcite ribbons and occasional spheroidal objects or “blobs” found in fine-grained, probably marine sediments spanning the late Archean to late Neoproterozoic interval, a duration of ~1900 m.y., or nearly half Earth's stratigraphic record. Vertical ribbons, averaging 5 mm in thickness, are generally intricately folded or fragmented by compaction. Decompaction shows that some ribbons may have been up to a meter in length perpendicular to bedding, forming in sediments that originally contained ~70% water, implying formation in sediment depths of ~1.7 m or less. Though at least 10 processes have been suggested for the blobs and ribbons, they were most likely voids created by the rise of gas bubbles from the decay of microbial mats that were rapidly filled with calcite. Horizontal ribbons probably formed under conditions of high pore-fluid pressure due to an overlying seal attributed to extracellular polymeric substances (EPSs) associated with the microbial mats. Cathodoluminescence imaging reveals that several molar-tooth fills show two distinct components: (1) earlier granular cores (now calcite) that are overgrown by (2) polygonal calcite. The earlier cores may have begun as an amorphous calcium carbonate (ACC) phase that recrystallized as calcite. Molar-tooth structures appear to simply reflect the formation of compacted calcite-filled voids under a seal of microbial mats and EPSs. Molar-tooth structures were present on Earth and may have played a significant role in the earlier history of other rocky extraterrestrial bodies with aqueous environments.