Twin stacking faults have been observed in vaterite tablets of freshwater lacklustre pearls for the first time. The detailed characteristic of stacking faults was studied by high-resolution transmission electron microscope (HRTEM) with selected area electron diffraction (SAED). It is found that stacking faults along [0 0 1] mainly consist of micro-twins with the twin plane (0 0 1). The stacking faults in vaterite crystals may lower energy for crystal growth, and at the same time make vaterite crystals more stable than the other polymorphs of calcium carbonate. At last, the formation mechanism of stacking faults was discussed. (C) 2007 Elsevier B.V. All rights reserved.