卵 ('egg') + 石 ('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 ('goose-egg stone'), the larger, more recognizably egg-shaped river stones used for paving traditional courtyards and footpath massage trails 健身步道.
