がん
いわ
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A mountain stone is probably going to be a really big stone. We're talking about a boulder here.
Although it doesn't share the kanji, this boulder shares the reading of the first part of the famous "Iwajima" (where a big battle in WWII took place). Think of a big boulder. Now think of it plopped into the ocean to make an island. What island is it? It's Iwajima (the historical spelling of Iwo Jima).