Tafsir of Ad-Dhariyat 51:33

(That We may send upon them stones) — that is, after overturning their villages, making their highest part their lowest, as has been detailed in the other noble chapters — (of clay) — meaning, petrified clay, which is al-sijjil. By qualifying them as being from clay, the false assumption is dispelled that they might be hail, for some people call hail "stones."