Lead dioxide, forming black, dense masses or crystals, an alteration product of lead minerals. It's associated with grounding and stability but is toxic.
Iron-Black, Brownish-Black, often forms crystalline crusts or massive
Plattnerite is classified as a oxide minerals mineral which typically crystallizes in the tetragonal system. Its chemical formula is PbO2. Common crystal habits include prismatic crystals, may be nodular or botryoidal, fibrous and concentrically zoned, massive.