I use mild steel on raw agate, like a 10d nail. I also use steel ice picks for notching, on raw or cooked. I try to keep everything sharp but not too sharp.
Cooked agate responds well to copper and antler but I don't cook mine. The results are unpredictable and I've lost some nice rock. I get my cooked agate from a friend. He's a master at cooking Brazilian agate.