Be sure to follow the rule before you head out.
Rule: You must post before you leave.
My geologist coworker said that agate isn't really a mineral either, mostly because it gets used too broadly and doesn't have one singular chemical composition.
Then again, I'm just an organic chemist, and classifying minerals seems suspiciously like biology to me...
I love this template, so many half-baked but quality rants
Wait, I'm confused. Glass is a blob that is slowly cooled, same with obsidian. https://en.wikipedia.org/wiki/Glass#Microscopic_structure
opal is what happens when spheres of particular size pack in grain structures of particular size.
you get like interference making diffraction patterns, but not monochrome because pseudo (meta?) crytals of balls modify the ball modes. you can make opal from latex balls, it's just not hard or as pretty. .
Basically opal is just the earth playing with its balls.