An Etymological Dictionary of Astronomy and Astrophysics

A fine-grained rock formed by the metamorphosis of clay, shale, etc., that tends to split along parallel cleavage planes, usually at an angle to the planes of stratification (Dictionary.com).

Etymology (EN): M.E. sclate, from M.Fr. esclate, feminine of esclat “piece split off,” (Fr. éclat) so called because the rock splits easily into thin plates.

Etymology (PE): Palmé “slate,” “a board on which children learn to read,” ultimately from Proto-Ir. *parn-, *parm- “feather,” PIE *pernom-, *pornos- “feather,” → tablet.