https://phys.org/news/2018-07-method-nucleation.html

Crystal nucleation is an inherently stochastic process, and estimating each nucleation time requires growth models that work backward from the time at which the crystal grew to a detectable size. This temporal gap can range from several minutes to hours.

"It's like saying when someone became pregnant once the baby is outside," said Romain Grossier, an author of the paper at the French National Centre for Scientific Research and Aix-Marseille University. "We detect the moment of pregnancy."