go look for calcite it is growing everywhere around you.

these beauties were growing on a shale outcrop just south of Ashokan Resevoir in NY.

 

These were growing on the face of a very metamorphosed road cut just west of Willmington VT on Rt. 9.

 

 A warty calcite crust with a green layer inside it. never did get around to crushing some under the microscope to see what was growing in there. was the green biofilm growing the calcite? growing on a limestone outcrop in Rosendale NY. downtown Rt. 213

 

When and how and whether exposed or inside fractures and how long it took for all these calcite growths to grow, I have no clue. woud be cool to learn. I suspect the one with the green (living?) layer in it is contemporary.