17 Septaria, Facultad de Ingeniería, UNAM

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The Septaria is a sedimentary rock that gets its name from its distinctive appearance, divided into septa (walls, partitions). The Anglo-Saxons call this concretion "turtle shell rock" or "beetle rock" due to its rounded shape, similar to a turtle's shell or the hard exoskeleton of a beetle, with wrinkled surface. \nThe Septaria is mainly composed of calcite (calcium carbonate), by clays (aluminum and sodium silicates) and by iron and aluminum oxides and hydroxides. The formation process begins with an existing gel of aluminum oxides and hydroxides that hardens on the outside, while the colloid mass inside dries out, causing a contraction of the entire mass, which then breaks apart forming characteristic veins. Later, these are filled with rich carbonates solutions that crystallize cementing everything.