User blog comment:Emptylord/Custom Champion - Ballistic the Living Artillery/@comment-7709681-20150630171732/@comment-7481207-20150630185640

"-mancer" or "-mancy" actually refers to divination. So geomancy means divination with stones or earth. Why that suffix came to mean controlling said element is a bit odd, but is possibly due to divining with an element requires you to control that element.