שמות כד, י: כְׁמַעֲשֵה לִבְׁנַת הַסַפִיר וּכְׁעֶּצֶּם הַשָמַיִם לָטֹהַר
The Rambam in his Guide explains that the word לבנת (usually translated as whiteness) in this verse signifies transparency and not a white color.
The Sforno explains that the object referenced here is a person’s spirit of intellect which begins as a clear substance lacking knowledge just as the sapphire lacks color. Hence Moshe, Aaron and the Elders experience HaShem through a prophetic vision. They comprehended that their intellect and soul were superior to their material being just as the essence of heaven (spirituality) is clearer than the matter of heaven.
This is the explanation of the end of the verse “and like the very substance of heaven’s clarity.” Their human intellectual spirit was separate and superior to their physical nature, and this is the “substance of heaven’s clarity.”
