tetradymite
noun
te·trad·y·mite
tə̇‧ˈtradəˌmīt
plural -s
: a mineral Bi2Te2S consisting of a telluride and sulfide of bismuth, sometimes containing selenium, being pale steel gray in color and of a metallic luster, and occurring usually in foliated masses (hardness 1.5–2, specific gravity 7.2–7.6)
Word History
Etymology
Late Greek tetradymos fourfold, from Greek tetra- + didymos twin; from its occurrence in compound twin crystals