Put a little pressure and heat on rock, give it time, and shale turns into slate. It’s the same with calcium carbonate slowly reinventing itself as chalk.
Limestone’s in no hurry; it started to harden during the Lower Jurassic into marble. Graphite spends millennia on diamond:
The luxury of eons. ………………………………At any rate, slow or slower, they move in mineral time with plenty of leisure for maturing late. Nice for them. I have a different clock, skin-shallow. Animals can’t afford to wait.