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Never really dawned on me this until I saw this in-game for myself
It looks like Steelix is chipping when it mega evolves
This clicked for me when I mega evolved my shiny and the debris orbiting Steelix is golden as well. The debris is actually the skin that has already chipped off.
The bullseye patterns now make sense if they are suppose to be geodes.
One of my favorite things about megas getting revealed is getting absolutely pissed at their designs, and then slowly come to love them as each detail added starts to make sense.