Boulder
A mountain stone is probably going to be a really big stone. We're talking about a boulder here.
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いわ
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"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.