noun
Refers to a small stone, typically one that can be picked up or thrown. Common in everyday contexts.
道に小石が落ちていた。
A pebble was lying on the road.
子供が小石を拾って川に投げた。
The child picked up a pebble and threw it into the river.
Compound of 小 (small) and 石 (stone). The reading こいし follows standard kun'yomi.