Al-jazʿ is a variegated stone often marked with dark and pale bands. Al-mufaṣṣal baynahu describes beads separated from one another by interposed pieces so that their colors and outlines remain distinct.
The wild cattle are not simply uniform white. Dark markings on portions of their bodies contrast with pale areas. Hence the necklace comparison conveys alternating colors in an ordered row, not merely attractive decoration.
The opening verb fa-adbarna also indicates turning away: the pattern is observed while the herd is moving.