Sforno on Exodus 24:10/ספורנו על שמות כ״ד:י׳

1ותחת רגליו, on the earth, the lowest point. We have G’d on record as describing “earth” in such terms in Isaiah 66,1 והארץ הדם רגליו, “the earth is My footstool.”

2כמעשה לבנת ספיר, an essence, totally transparent, devoid of colours and permanent contours so that it is almost completely abstract, capable of absorbing spiritual input from spiritual domains at will. An allegorical description of the human נפש, “life-force.”

3וכעצם השמים לטוהר, they perceived that this לבנת הספיר, “essence,” is totally independent of the raw material man is made of; just as what we perceive as the essence of heaven in the sky is totally devoid of tangible matter. It is the essence of what we consider the celestial regions, “heaven,” for lack of a better word. It is not connected to the solid physical planets etc., which form the “inhabitants” of these celestial domains. The fact that the “sky,” “heaven,” has no physical dimensions makes it appear as if it is the spiritual dimension of “Heaven” itself.