An earth-boring bit of the fixed cutter variety is provided with hard gage inserts that protrude from the gage surface of the cutter to engage the side of the borehole for holding gage. The gage insert has a substantially flat, polygonal face defining at least a pair of sharp cutting edges and at least a pair of cutting surfaces that define a negative rake angle with respect to the sidewall of the borehole that is being sheared by the gage insert. The face, cutting edge, and cutting surface of the gage insert are formed of a super-hard and abrasion-resistant material such as polycrystalline diamond or cubic boron nitride. The body of the insert is formed of a hard, fracture-tough material such as cemented tungsten carbide. The improved gage inserts provide an actively cutting gage surface that engages the sidewall of the borehole to promote shearing removal of the sidewall material. Such an improved gage insert provides an earth-boring bit with improved gage-holding ability, and improved steerability in directional drilling operations.