ὁπλαὶ δέ σφισίν εἰσιν ἐλέφαντος
Their hooves are made of ivory.
The genitive signals a material or contents as a predicate NP.
τὸ δὲ ἄγαλμά ἐστι χρυσοῦ καὶ αὐτοῦ τὸ πρόσωπον ἀντὶ ἐλέφαντος ἵππων τῶν ποταμίων ὀδόντες εἰσὶν εἰργασμένοι
The image is of gold and its face is not of ivory, but consists of wrought hippopotamus teeth.