Refers to a stone or rock, often rounded by water, ranging from small pebbles to larger boulders. The size is context-dependent.
I picked up a round pebble at the riverbank.
I placed a large boulder in the garden as a decoration.
Compound of 玉 (たま, 'ball, jewel') and 石 (いし, 'stone'), referring to a stone that is rounded like a ball. The reading たまいし is a straightforward combination of the two elements.