Semseyite is a rarely occurring sulfosalt mineral and is part of the class of lead antimony sulfides. It crystallizes in the monoclinic system. The mineral forms dark gray to black aggregates. Semseyite can be found in mid-temperature hydrothermal veins in localities including Romania, Germany, France, Scotland, Bolivia, Peru, the USA, Canada, and Japan as well as in small amounts at other occurrences.