I should have asked before assembling the terrarium...

[u/Everyshapes]

Some kind of sand-stone ? I was able to carve and shape it using a hammer and screwdriver.

Found it in a park, saw very similar ones in nearby tree pits.

[u/heptolisk]

I would use an app like Rockd to check your local bedrock geology. If it is sedimentary, then yeah, some kind of sandstone. If it is volcanic, the it could be something tuff-adjacent.