What are these "stones" mentioned in the verse referring to?

Imām Ibn al-Jawzī (d. 597 AH / 1201 CE) summarized the scholars' opinions in his book "Zād al-Masīr":

1. They refer to the idols they worshipped. [al-Rabīʿ ibn Anas]
2. They refer to sulfur stones, which are among the hottest materials when heated, and they will be used to punish them.