Boulder
A mountain stone is probably going to be a really big stone. We're talking about a boulder here.
This kanji makes a lot of sense. Just follow the logic.
がん
いわ
None
"Boulder!!!" yells a voice from behind you. "Eek, wha...?" (いわ) you reply. You look up to see a boulder falling off the cliff above you. "Eeeeeek! Whaaaat!?," you shout again, running and screaming this time.
Imagine yourself screaming "Eeeeeek! Whaaaat!?" and running away from that boulder as fast as you can.