A grinder for plant substances includes a first surface and an opposing second surface that is separated from the first surface by a first longitudinal gap so that the first surface and the second surface define a receptacle within which a plant substance may be received. The second surface includes holes through which ground pieces of the plant substance may pass and at least one of the first surface and the second surface includes bumps that extend into, but not across, the receptacle. The bumps encourage rolling friction between the plant substance and the bumps so that the plant substance is ground into pieces small enough to pass through the holes.