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Well done ! It was in Tasmania. The rock is dolerite (they call the same rock diabase in the US). @r00sj3 chose somewhere in Tassie first so you get a 2nd place 40% of the prize !

I'll back your guess, because it's the most unique one! Most people are thinking North America...

Well done although @jarvie may have backed Tassie first!

Oh, wow, it actually WAS Tasmania! :o