Lit., "stones of sijjil", which latter noun is regarded by some philologists as the Arabicized form of the Persian sang-i-gil ("clay-stone" or "petrified clay"): cf. Qamus and Taj al-'Arus. If this supposition is correct, the "stones of petrified clay" would be more or less synonymous with "brimstones", which in its turn would point to a volcanic eruption, probably in conjunction with a severe earthquake (alluded to in the preceding phrase, "We turned those [sinful towns] upside down"). But there is also a strong probability, pointed out by Zamakhshari and Razi, that the term sijjil is of purely Arabic origin - namely, a synonym of sijjil, which primarily signifies "a writing", and secondarily, "something that has been decreed": in which case the expression hijarah min sijjil can be understood in a metaphorical sense, namely, as "stones of all the chastisement laid down in God's decree" (Zamakhshari and Razi, both in conjunction with the above verse and in their commentaries on 105:4). It is, I believe, this metaphorical meaning of "stone-hard blows of chastisement pre-ordained", i.e., of God-willed doom, that the concluding sentence of the next verse alludes to.