Word for start (like catalyst) of something forming

Question

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.

Answer 1

For the first part of your question;

The imperfections on a surface where air bubbles form in a liquid are called nucleation points. When referring to a crystalline structure forming it is called nucleation.

As defined by Britannica:

Nucleation, the initial process that occurs in the formation of a crystal from a solution, a liquid, or a vapor, in which a small number of ions, atoms, or molecules become arranged in a pattern characteristic of a crystalline solid, forming a site upon which additional particles are deposited as the crystal grows

For the second part of your question;I would suggest stimulate

1.2 - Encourage development of or increased activity in (a state or process)

Answer 2

From a different branch of science altogether, germ and its associated words germinate and germination spring to mind. In its figurative meaning:

[SINGULAR]

something that could develop into a greater idea or plan

the germ of an idea

From http://www.macmillandictionary.com/dictionary/british/germ

Answer 3

Incipient

just beginning: signs of incipient public frustration

Cambridge

As for the first part of your question, as Skooba pointed out, I believe the things you describe are called nucleation sites.