Аль-Филь 105:4

Verse [ 105:4] بِحِجَارَ‌ةٍ مِّن سِجِّيلٍ (...stones of baked clay.) The word sijjil is a compound Persian word, Arabiciszed, made up of two Persian words: sang [ stone ] and gil [ clay ]. They refer to ordinary or common clay rolled into little balls and then baked to harden. These pebbles are not heavy or hard like rocks chipped off mountains. They in themselves did not have any devastating power, but it was Allah who created in these stones the high capability of explosion and blowing-up even more than the bullets of a revolver.