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The deep-red lenses of the Pragian Slivenec Limestone have been extensively quarried for ornamental purposes since the XIIth century. Petrographic microscope observations indicate that the hematite stainings of the limestone follow ten different patterns. They range from massive non-directional filling of cavities to mineralized films and microstromatolites. Numerous iron-rich endolithes are observed. Some could be derived from bacterial or lichen perforations and some related to ferric bacteria. Infiltration along welded calcite crystals, regular mineralized films and microstromatolites suggest a ferric bacterial origin for the pigment. This is confirmed by scanning microscope examinations of polished sections, that show hematite concentrations along micrometric filamentous sheaths.