Coal scrubber residue.
An example of the element Sulfur

Sulfur from a desulfurization unit at a southwestern Pennsylvania coke plant. Specimen is approximately 7.9 x 2.2 x 1.0 cm. Sulfur is recovered from coke oven gas by reacting hydrogen sulfide with sulfur dioxide. Nearly pure elemental sulfur results from this environmentally friendly process. Sulfur stalactites and stalagmites form around leaking valves.