The formation of ice crystals have important implications for stratospheric ozone chemistry, cloud dynamics, rock weathering, and hydrate formation etc., however the exact mechanisms by which microscopic particulate matter 'seeds' ice-crystals are unknown. Atmospheric dust (clay dust etc) and microscopic organisms are known to seed clouds, but there is currently no explanation as to why some varieties are very effective and others are not. 1)
Cirrus clouds for example, which cover about 30% of the Earth’s surface, are formed from ice crystals, the majority of which have been seeded by a particle (in the process known as heterogeneous nucleation).
“Cirrus clouds play an important role in the radiative budget of the Earth. Due to scattering and absorption of the solar as well as terrestrial radiation the cirrus cloud cover may influence significantly the Earth climate. How the cirrus clouds exactly form, is still unknown.”
“How aerosol particles act as nuclei for ice and other crystalline phases like hydrates, and how this can quantitatively be described in models, are among the major open questions in atmospheric cloud and climate research.”
Note: The question also arises in the formation of snow flakes, which are also often 'seeded' with microscopic particulate matter at their centre. Exactly how the particles facilitate the flakes to form is unknown.