What's the word that could best describe the sound of a pebble (or pebbles) falling on a rock surface? Clack, tap, clank...? Thanks!
A continuous rattling of hard objects striking each other is often described onomatopoeically as "clattering". The term is well suited for either multiple pebbles or multiple contacts of one pebble with a rock. A single controlled strike of a single pebble would probably be better described as a "click" (for a higher, sharper noise, if the pebble and rock are both smooth, for example) or a clack or tap, as you wrote. "Clank" generally implies metal rather than stone.