boulder; cobble; round stone
A naturally rounded stone, often found in rivers or used for decoration. Can refer to stones of various sizes, from small cobbles to large boulders.
I found a large round stone on the riverbank.
I laid round stones in the garden to make a path.
Compound of 丸 (まる, 'round') and 石 (いし, 'stone'). The reading is a straightforward kun'yomi combination.