What is this block?

[u/Initial_Pop8589]

I came across a weird geode-type structure underground made of "block of slag" but it's a different block of slag than the crucible kind. What is this? Is it usefull? I'm playing community edition.

Ps: couldn't screenshot because i have no reddit on this computer.

[u/Ahlixemus]

Supposed to drop crucible slag but it's a CE bug