Wow. There's really not much information on Fearless Flint on the internet. So I don't know much about him. He first appeared in FAMOUS FUNNIES #89 (December 1942). And he was drawn by Wonder Woman artist Harry G. Peter.
International Hero adds: While helping excavate a mountainside near Mount Rushmore, construction worker Jack Bradley was caught in an explosion and resultant avalanche, and covered in a mysterious dust. Afterwards he discovered he could turn into flint whenever he became angered.
Although here it seems he's also "powered up" by touching metal...
That's about it!!
Here's a story: