Jamieson, Fausset and Brown on Exodus 24:10

10. And they saw the God of Israel--That there was no visible form or representation of the divine nature, we have expressly intimated (De 4:15). But a symbol or emblem of His glory was distinctly, and at a distance, displayed before those chosen witnesses. Many think, however, that in this private scene was discovered, amid the luminous blaze, the faint adumbrated form of the humanity of Christ (Eze 1:26; compare Ga 3:24).

sapphire--one of the most valuable and lustrous of the precious gems--of a sky-blue or light azure color and frequently chosen to describe the throne of God (see Eze 1:26; 10:1).