Crystal nucleation in polar stratospheric clouds (PSCs) has a direct impact on stratospheric chemistry and ozone. However, the mechanism of nucleation has been unclear for decades, limiting prediction of the response of ozone to atmospheric changes. We experimentally demonstrate that meteoric material can trigger nucleation heterogeneously and this can produce observed crystal concentrations in PSCs. This discovery paves the way to robust modelling of past and future trends in PSCs and ozone.