Shale. Credit: Mineral Information Institute.
Shale is a common sedimentary rock produced from clay, which has a fine-grained structure in well-defined narrow strata, 0.1–0.4 millimeter (0.004–0.016 inch) thick, each representing a period of deposition. Shales are rich in fossils, and may contain carbonaceous matter and oil. Their metamorphism produces slate.