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Actually not all rocks are solid lava.. I mean they weren't all formed from cooled lava... Most are actually made by compression. Igneous rocks are the ones from lava..
I remember this all from 8th grade science
If one REALLY wants to bend obsidian, add it to the list of bendable blocks. Obsidian is literally just rock, not actual lava, it wouldn't make sense to disallow it from non-lavabenders. Denied