Whetting is the act of rubbing an edge to sharpness with an object such as a stone. Whetstones are naturally quarried or synthetically made, either way they are a cohesion of abrasive particles. Those used for honing and sharpening European scythe blades are canoe shaped to accommodate the multi-plane curvature of the blades.
Stones are continually dipped in water during use which creates a slurry of fine particles for sharpening and prevents overheating of the steel. Stone pores clog with dirt and debris over time; a 1-hour soak in a solution of 1 part white vinegar to 6 parts water will help clear the pores and renew the toothiness of the stone.
Whether synthetic or natural, stones wear as they're used. The matrix holding the stone particles together breaks down, particles swim away in the slurry. Over time natural variations in the stone or in the sharpener's pressure and rhythm can turn a symmetric canoe shape into a knobbly witches finger of a thing. Coarser stones break down faster because particles are larger so there are fewer "layers" to peel away.
And, if you drop it on concrete, well then you have two!