smut
n.n. a type of soft, dark rock that is easy to break and is often found in coal mines. It is usually made of clay and other minerals.
n. a soft, dark, brittle rock consisting of clay, sand, and other minerals, typically found in coal seams. Often used in the mining industry to describe material that is difficult to remove from the coal.
The miners had to clear the coal of smut before it could be sold.
The coal was contaminated with a thick layer of smut, making the processing plant's filters work overtime.
Geologists noted that the presence of smut in the seam indicated a high level of pressure and rapid sedimentation during the formation of the coal deposit.
From Middle English smutten (“to defile, debase”), related to German Schmutz (“filth, dirt, smut”) and schmutzen (“to make dirty, stain”). Doublet of schmutz. Compare also Old English smitta (“smear; blot; mark; stain; pollution”), Old English besmītan (“to besmut; defile; dirty; pollute; contaminate”).