Targum su Esodo 24:10

And Nadab and Abihu lifted up their eyes, and saw the glory of the God of Israel; and under the footstool of His feet which was placed beneath His throne, was like the work of sapphire stone a memorial of the servitude with which the Mizraee had made the children of Israel to serve in clay and bricks, (what time) there were women treading clay with their husbands; the delicate young woman with child was also there, and made abortive by being beaten down with the clay. And thereof did Gabriel, descending, make brick, and, going up to the heavens on high, set it, a footstool under the cathedra of the Lord of the world whose splendour was as the work of a precious stone, and as the power of the beauty of the heavens when they are clear from clouds.

The footstool of His feet as the work of pure sapphire stones, and as the aspect of the heavens when they are cleared from clouds.