Shale is the protolith of slate.

Rock group	Sedimentary
Origin	Clastic/detrital
Particle size Wentworth	≤ 62.5 µm
Special properties	Fissile; splits along bedding planes

Shale in Hand Sample

Shale is often fossiliferous, frequently preserves sedimentary structures such as crossbedding and mudcracks, and comes in varying shades of grey, tan, black, green, and red.

Fossiliferous Millboro Formation shale from West Virginia
Mudcracks and raindrop impressions in a red shale
Graptolites in black shale
Argillaceous shale

Shale in Thin Section

Laminated shale, plane polars
Laminated shale, crossed polars