1747, possibly a specialized use of M.E. schale
"shell, husk, pod" (c.1380), also "fish scale," from O.E. scealu
) in its base sense of "thing that divides or separate," in ref. to the way the rock breaks apart in layers. Geological use also possibly influenced by Ger. Schalstein
"laminated limestone," and Schalgebirge
"layer of stone in stratified rock."