I spent years calling all dark, shiny rocks 'obsidian' until a park ranger in Yellowstone corrected me
I was showing off a piece of what I thought was obsidian I found near a creek, and the ranger said, 'That's not volcanic glass, that's just a really smooth piece of chert.' He pointed out the conchoidal fracture was different and it didn't have the same sharp edge. I felt pretty silly, but it made me actually learn how to tell them apart. Anyone else have a basic rock ID they were totally wrong about for way too long?