So is iron. At various temperatures iron changes its crystal structure from body-centered cubic to face-centered and back again to BCC and the amount of carbon

So is iron. At various temperatures iron changes its crystal structure from body-centered cubic to face-centered and back again to BCC and the amount of carbon that dissolves varies in each crystal structure, and you can hack the system by shock cooling to control the hardness of steel over a ridiculous range. That's a one line metallurgy degree, or "this chemistry fact resulted in the industrial revolution". Water is also unstable, I can't keep track of how many non-hexagonal ice structures exist under some weird pressure and temp diagram. 12 or so, maybe. You put ice under a couple hundred MPa and it gets all funky. On the topic of allotropes its depressing when you think everything out there has allotropes except thankfully maybe the nobel gasses like helium and then oh God there's liquid helium-II which isn't really an allotrope but its conceptually similar. It seems that most of learning chemistry is learning nothing is stable and nothing is as it superficially seems. Plutonium is scary creepy but for better or worse its not unusual.