Maybe my mind is playing tricks on me. I thought there was a word for the "imperfections" where water bubbles form in a glass. Or the instigation of the nucleus forming for a crystal formation. Something like catalyst, but not chemical. But I'm drawing a blank.
I'm trying to capture (without being verbose) the idea of something that is going to start, but is waiting for a trigger. Not in hindsight (X was the catalyst for Y), but to express something impending to people that aren't expecting a change.