The magnetorheological material compositions of the invention include an aqueous carrier fluid, magnetic-responsive particles, and bentonite or hectorite. The aqueous carrier fluid preferably makes up between about 10 and 50 percent by weight of the composition. The magnetic-responsive particles preferably make up between about 50 and 90 percent by weight of the composition. The bentonite or hectorite preferably makes up between about 0.1 and 10 percent by weight of the composition. The fluids typically develop structure when exposed to a magnetic field in as little as a few milliseconds. The fluids can be used in devices such as clutches, brakes, exercise equipment, composite structures and structural elements, dampers shock absorbers, haptic devices, electric switches, prosthetic devices, including rapidly setting casts, and elastomeric mounts.