ToME Wiki
General Info
Currently only Ents have this ability. I, MrConceit, haven't USED it as such (for offense), but looking at the information on Killerbunnies, the damage appears respectable. A side-effect is that you have a zero cost instant ability to tunnel in walls with this skill.
Code-related Information
If you have a skill of 1 or greater you can create boulders from granite walls via the option in the 'm' menu. A thrown boulder does damage of (5d5+(Skill*80/50))*(Skill*6/50). Plugging in numbers to skill level 50, this gives us an average damage of 95*6, or 570. The range of a boulder is ((R+20)*(10+2*(Skill*6/50)))/50 to a maximum of 10+2*(Skill*6/50), where R is a STR-based adjustment. The hard coded max looks to be 22. I'll leave it to the reader to bother to figure out their exact range, but you can go to www.killerbunnies.org if you want to see the STR-based chart.