Never said he was gonna make Dragon armor out of Dragonglass.Dragon Glass is just obsidian and is too brittle to use as armor.

All I said was that Gendry is ‘no joke’ when it comes to FORGING armor & weapons... because he led a bunch of civilians in crafting thousands of weapons in a very short time.

The man knows how to work.... at scale... and make thousands of pieces quickly.
Not one at a time.
And dragon armor would be made of lots of ‘small pieces’ of armor to form scales. So I’m pretty sure Gendry could pull it off.
