Where does “शैल” come from?

शैल (Sanskrit) comes from Sanskrit शिला, from Proto-Indo-Iranian *ćHláH.

Ancestry of “शैल”, step by step

Step	Language	Word	Meaning
1	Sanskrit	शिला	stone, rock, crag; red arsenic; camphor
2	Proto-Indo-Iranian	*ćHláH	—

शैल — Etymology, Origin & Word History | Etymology Explorer