Shale

Shale is a type of rock that can be found underground and occasionally above ground. It may be more common around freshwater rivers. Along with Sandstone, it is one of 2 rocks that can be broken by players at the start of the game with a Stone Pickaxe and no Mining talents.

Shale seems to be most commonly found in the Rainforest biome, but may be found in other biomes. It is usually close to the surface.