When Moses, Aaron and seventy elders saw the God of Israel, what was under His feet?

Answer: A pavement like sapphire

Under His feet was "a work like a pavement made of sapphire, as clear as the sky itself" (KJV: "paved work of a sapphire stone").

“Then Moses went up with Aaron, Nadab and Abihu, and seventy of the elders of Israel, and they saw the God of Israel. Under His feet was a work like a pavement made of sapphire, as clear as the sky itself.”— Exodus 24:9-10 (BSB)