UnScramble this Word: a h e t n r?

Questions

Geology : There are three types of rocks: metamorphic, sedimentary, and _________.?

Geology : These limestone deposits rise from the floor of caves.?

Geology : This is the hardest naturally occurring substance.?

Geology : What name is used to describe permanently-frozen subsoil?

Hahaha : This normally has 4 legs and your butt is parked in it right now.?