7 Photos
Tetradymite
Chemical Formula	Bi2Te2S
Species	Sulfides
Crystal System	Trigonal
Mohs Scale	1-2
Specific Gravity	7.2-7.9
Color	Steel-gray with dull to iridescent tarnish; white in polished section
Streak	Steel-gray
Luster	Metallic, Dull
Diaphaneity	Opaque
Cleavage	Perfect on
Fracture	Uneven
Crystal Habit：Pyramidal prisms, commonly granular, massive to foliated, also bladed

Tetradymite is a mineral consisting of bismuth, tellurium and sulfide, Bi₂Te₂S, also known as telluric bismuth. If sulfur is absent the mineral is tellurobismuthite and the formula is then Bi₂Te₃. Traces of selenium are usually present.

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-grey, and the lustre metallic and brilliant. The mineral is very soft (H = 1.5 - 2) and marks paper. The specific gravity is 7.2 to 7.9.

The type locality is Zupkov (Zsubko; Schubkau), Stredoslovensky Kraj, Slovak Republic where it was reported in 1831. It was first found, in 1815, at Telemark in Norway. It often occurs in high temperature hydrothermal quartz veins associated with native gold and in contact metamorphic deposits.