Slate

i. A fine-grained rock displaying strong slaty cleavage and hence can be split into slabs and thin plates. Most slate was formed from shale during low grade regional metamorphism. Usually gray to black, sometimes green, yellow, brown, or red. Slates are composed mostly of quartz, micas, chlorite, with minor hematite, clays, and other minerals.

Ref: AGI

ii. A coal miner's term for any shale accompanying coal; also, sometimes the equivalent of bone coal.

Ref: AGI