schist 
(shĭst)
(shĭst)n.
Any of various medium-grained to coarse-grained metamorphic rocks composed of laminated, often flaky parallel layers of chiefly micaceous minerals.
[French schiste, from Latin (lapis) schistos, fissile (stone), a kind of iron ore, from Greek skhistos, split, divisible, from skhizein, to split; see skei- in the Appendix of Indo-European roots.]
schistose′ (shĭstōs′), schistous (-təs) adj.
schist