Tetradymite is a mineral consisting of bismuth, tellurium and sulfide, Bi<sub>2</sub>Te<sub>2</sub>S, also known as telluric bismuth. If sulfur is absent the mineral is tellurobismuthite and the formula is then Bi<sub>2</sub>Te<sub>3</sub>. Traces of selenium are usually present.
left|thumb|180px|A sample of tetradymite
Crystals are rhombohedral, but are rarely distinctly developed; they are twinned together in groups of four; hence the name of the mineral, from the Greek for fourfold. There is a perfect cleavage parallel to the basal plane and the mineral usually occurs in foliated masses of irregular outline. The color is steel-gray, and the luster metallic and brilliant. The mineral is very soft (H = 1.52) and marks paper. The specific gravity is 7.2 to 7.9.
The type locality is Zupkov (Zsubko; Schubkau), Stredoslovenský Kraj, Slovak Republic where it was reported in 1831.