A friable, degraded form of coal that is heavily mixed with clay, dirt, or other earthy materials. It is often found near the surface where weathering occurs, or adjacent to geological faults where tectonic movement has crushed the seam and introduced impurities. Because of its high ash content and low calorific value, it is generally considered waste.
Heard on the job
The seam starts turning to smut as you get closer to the fault line.