If the phrase is taken to mean "the earth's corners or quarters", then the image is of the four arms of the cross, each ending in a jewel stretching from horizon to horizon. See Images: Ravenna: S. Apollinare in Classe for an image of a jewelled cross stretching across a background of stars. However see the discussion in Mahler 1978 esp. p.446ff. where she suggests the image is of a processional cross surmounting a jewelled sphere (= the world), mounted on a staff.