A hardened turd works best.

 

Of course, you could always use a diamond tip drill bit/engraving drill bit.
Seen people use pumice rock, volcanic rock, said they got it from a place that sold it for fish tanks.
I'd go with the hardened turd though.

 

Sharpening rocks.. or drill bits that are used for grinding.

 

really anything sharp but i use drill bits screws ect.