pebble

这块鹅卵石有着独特的纹理。

This ___pebble has a unique texture.

我在河边捡到一颗光滑的鹅卵石。

I found a smooth ___pebble by the riverbank.

他踢起一块小石子，看着它滚下山坡。

He kicked a ___pebble and watched it roll down the hill.