Malbim: It is mentioned regarding Chayos, because they are not physical. One does not see them with his eyes, just they appeared to him in a vision. The Ofanim are physical; the eye sees them.

Rashi: It is like a gem called Tarshish (crystal).

Radak: Tarshish is one of the gems on the Choshen - "Tarshish Shoham v'Yashfe" (Shemos 28:20). In the third vision, it says "k'Ein Even Tarshish" (below, 10:9). Its color is like Techeles, which is like the sea (Menachos 43b), i.e. Yam Tarshish, and the sea is like the Raki'a.

Radak: Acts and work are done to them, like to Tarshish.

Malbim: Tarshish is like Techeles. The thin air through which the stars go, it appears to the eye like Techeles. It says "Yadav Gelilei Zahav Memula'im ba'Tarshish" (Shir ha'Shirim 5:14).

Rashi: The Ofan (wheel) is fixed in the Ofan vertically and horizontally, for they go in all of the four directions ¹ . Therefore, the wheel must roll in four directions.

Malbim: A wheel fixed in a wheel moves in every direction. The daily orbit of all the Galgalim is from east to west. Each star has its own movement in its special time, from west to east ² . It is not on the equator. Rather, each star leans to the north or south. For six months it goes from north to south, and for six months from south to north. The sun goes from Mazal Gedi to Sartan, and returns to Gedi. It veers about 47 degrees ³ . The moon's extremes are more than five degrees from its medium place. Every star has its head and tail (southern and northern extremes); it goes in every direction, like a wheel fixed in a wheel.