This is a localized term for a low-quality band of coal contaminated with iron pyrite, shale, or calcium carbonate. Because of its high impurity content, it burns poorly and produces excessive ash or sulfur dioxide. Miners usually have to separate and discard this material during extraction to avoid downgrading the quality of the run-of-mine coal.
Heard on the job
The cutter hit a thick band of brat, so we'll have to sort that out before it hits the main belt.