The source is, naturally, the Quran. The 105th chapter is called Sūrat al-Fīl, The Elephant.
It describes the Ethiopian army attacking Mecca in the year 570, the year in which Muhammad was born. In verse 4 it says that Allah sent birds against the people/owners/companions of Elephants (depends which translation of the Koran you listen to), a reference to the Ethiopian army. These birds dropped "sijjil stones" on the Ethiopian army - most English translations translate this as "baked clay stones", as can be seen here:
Have you not seen how your Lord dealt with the People of the Elephant?  Has He not turned their plan into nullity?  And He sent upon them flying birds in flocks,  throwing upon them stones of baked clay,  and thus He turned them into an eaten-up chaff.
Here's an unintentionally funny video illustrating the chapter.
Although the translation "Shale Stones" is the exact translation, the Arabic "Hijjarat As-sijjil" implies this Quranic meaning.
Gawker and "Jewish Voice for Peace" managed to find nefarious, offensive meanings in the Hebrew title that is - as any Jewish schoolchild knows - a reference to a story where the Israelites are saved by a miraculous cloud that defends them.
Will they have anything bad to say about Hamas' clear choice of naming their war after an episode where Allah attacks and destroys the enemy with projectiles until the adversary is nothing but chaff?
(thanks to Al-Gharqad)