Polar Stratospheric Clouds (PSCs) catalyze ozone depletion at high latitudes with important impacts on energetic, biologically effective UV radiation. The present study is a unique experiment into PSC particle optical properties and formation performed in a large, coolable cloud chamber. Robert Wagner and his colleagues find evidence that a nitric acid dihydrate can be efficiently crystallized in PSCs through heterogeneous nucleation, for example by mineral dust or micrometeorites. It has a "beta-polymorph" crystalline structure - with distinct optical properties - clearly separable from an alpha-polymorph that forms at low temperatures through homogeneous nucleation. Interestingly, electron microscopy, infrared spectroscopy, X-ray diffraction and calculations all point to the same conclusion. The results have important implications for our understanding and the detection of PSCs.