FIG. D-26. Drill bits. (a) Drag bit or fishtail bit; the teeth on drag bits tear into soft formations like sand and clay as the drill stem is rotated. (b) Rock bit or roller bit; teeth on rock bits are on wheels that turn as the drill stem is rotated, so that they alternately put pressure on the rock and relieve the pressure, which causes rock pieces to flake off. (c) Diamond bits containing diamonds embedded in the bottom can be used in very hard formations.