卵 (luǎn, 'egg') + 石 (shí, 'stone') = 'egg-stone' — pebbles smoothed and rounded by water until they look like eggs. The image is exact: that's literally how Chinese describes the shape.
usage
Distinguish from 鹅卵石 (éluǎnshí, 'goose-egg stone'), the larger, more recognizably egg-shaped river stones used for paving traditional courtyards and footpath massage trails (健身步道, jiànshēn bùdào).
