Blister

1. In quarrying, an unconfined charge of explosive used to bring down dangerous ground that cannot be made safe by barring and that is too inaccessible to bore. South Australia
2. A protrusion, more or less circular in plan, extending downward into a coal seam. It represents the filling of a streambed pothole worn into the upper surface of the coal-forming material. AGI
3. Copper as a smelter product before it is refined. Hoffman
4. A defect in metal, on or near the surface, resulting from the expansion of gas in a subsurface zone. Very small blisters are called pinheads or pepper blisters. ASM, 1

Source: Dictionary of Mining, Mineral, and Related Terms