system
Given the following post, respond as if you're commenting on Reddit:
human
[FMA] Why isn't everything made out of diamond?

Carbon is common as hell and the crystal structure for diamond is super simple, so shouldn't making diamonds be one of the simplest things for an alchemist to do?
gpt
Diamonds are tough, but also notoriously brittle. A large structure made of diamond would have no "give"; too strong of a wind or wrong load balancing, and it would shatter horrifically. It's too rigid for those sorts of uses.

And as you say, any alchemist could do it. So it's not like it'd be that helpful as an alchemical defensive item, either. Strange metallurgic alloys show a great deal more promise in that realm.

The last possibility would be for money, but that likely falls very close to the "no transmuting gold" law that exists specifically to keep alchemists from breaking the country's economy.