Polar stratospheric clouds (PSCs) of type 1a or 1a-enh containing high number densities of nitric acid trihydrate (NAT) particles, can act as mother clouds for extremely large NAT particles, termed NAT-rocks, provided the air below the clouds is supersaturated with respect to NAT. Individual NAT particles at the cloud base fall into undepleted gas phase and rapidly accelerate due to a positive feedback between their growth and sedimentation. The resulting reduction in number density is further enhanced by the strong HNO<sub>3</sub> depletion within a thin layer below the mother cloud, which delays subsequent particles. This paper introduces the basic microphysical principles behind this mother cloud/NAT-rock mechanism, which produces 10<sup>-4</sup> cm<sup>-3</sup> NAT-rocks with radii around 10<font face="Symbol">m</font>m some kilometers below the mother cloud. The mechanism requires neither selective nucleation nor additional atmospheric dilution and works even for a monodisperse particle size distribution in the mother cloud.