50How to craft it
Loomthe hat decides what it does
The robe is the bottom rung of the gem robe ladder, and on its own it is a shirt with no defense at all. What matters is the head slot. Pairing it with the Wizard Hat turns it into a critical strike set, while the Magic Hat turns it into a flat mana pool instead. Legs are free either way, so wear whatever you already have and keep the bonus.
the tim trick
Wear any gem robe in a real armor slot without a wizard hat on your head and Tim starts showing up far more often underground. That is the fastest way to farm him for his hat, and the only cost is the shirt slot while you tunnel. Put the wizard hat on and the effect stops, which is exactly backward from what most people try first.
when to stop wearing it
Zero defense is not a rounding error in pre-Hardmode. Every hit you take lands at full value, so this is a glass build that only works if you are already good at not getting hit. The moment you can craft a real magic set, the robe goes in the vanity slot.