Damaged and faulted speleothems have proven valuable for neotectonic investigations and dating. Flowstone (or “bacon”) speleothems often consist of coarse (cm-scale) columnar calcite with a pronounced primary crystal orientation. However, experimental and microstructural investigations of faulted flowstones are rare. What are the deformation mechanisms in flowstone and how can they be compared to deformation in faulted limestone and marbles is unclear. This study examines deformation mechanisms in flowstone and shows how crystal orientation affects pulverization. Direct shear experiments of columnar flowstone calcite were conducted under room conditions, employing low sliding velocities (1–10 μm/s) and normal stresses (3–10 MPa). The distinctive primary crystal orientation and relatively large crystal size (up to 10 mm) of flowstone allow for an intra- and intercrystalline deformation analysis. The deformation architecture and mechanical strength directly depend on the primary crystal orientation. Mechanical twinning increases in density and thickness, and it accommodates change in crystal orientation. Retrieved samples reveal in-situ pulverization with a drastic grain size reduction, caused by an interplay between mechanical twinning, cleavage and fracture propagation. At relatively small displacements (<10 mm) and strain (0.2) comminution develops a fine gouge. These findings contribute to a better understanding of the behavior of faulted speleothems and their potential implications for neotectonic studies.