Shale is a sedimentary type of rock. Sedimentary rock is formed when tiny particles of sand, pebbles or other fragments of material gather in layers, hardening over time.
Shale consists of silt and clay particles. It is the most common type of sedimentary rock. Shale is fissile, which means it has a tendency to break into tiny, thin layers.
The color of shale rock is consistent with the type of sediment it's composed of. If shale appears red or purple, the sample contains hematite and limonite. Samples of shale containing high levels of calcite appear light gray or yellow in hue.