I remember reading in one of John McPhee's geology books (collected as [1], and one of the best things I've ever read) about a particular way that diamonds can form; it had a name (like a diamond jet or a diamond burst or something) but the basic idea is mesmerizing: super-heated water containing dissolved carbon would find some fissure in the surrounding rock and explosively expand into it, and in the process it would cool rapidly and diamonds would crystallize out of the solution. I'm stuck with the visual of a seam of diamonds popping into existence in an instant, which is not what we normally think of as a geological timescale.

[1] https://en.wikipedia.org/wiki/Annals_of_the_Former_World