Brass. More than 5 mil thick.

If you squint, you might see a Jolly Wrencher in there. It didn't come out like I wanted, but the fault was mine.

After changing from 0.020 in (0.5 mm) end mill down to 0.010 in (0.25 mm) for details I set Z too low (because reasons + failed attempt to adapt) so the tiny mill cut a lot more than intended, right up until it crashed in the last 0.001% of the cut, proving that it absolutely can do the job. So I'm fairly confident that the next try will go better.

...reflecty...