polish
磋 centers on the act of polishing or refining a surface, especially by rubbing with an abrasive like stone. The meaning is straightforward and concrete.
磋 combines 石 (stone) with 差, which likely contributes the sound. The stone radical suggests polishing or grinding with a stone.
The left side is a stone 石, and the right side 差 looks like a hand holding a tool. Imagine using a stone to polish a rough surface until it shines.
For サ, picture polishing a surface with sand: sand -> サ, and the stone 石 helps you polish.