whetstone
硎 centers on a whetstone: a stone used for sharpening blades or tools.
硎 combines 石 (stone) with 刑, which likely contributes the sound and may relate to cutting or shaping, fitting a stone used for sharpening.
The stone 石 on the left and the sharp-edged shape 刑 on the right suggest a stone used to give a blade its edge: a whetstone.
For ケイ, imagine a blade being sharpened on a whetstone until it can cut through a cake: cake -> ケイ, and the stone gives the blade its edge.