Galena

Chemical Formula : PbS
Hardness : 2.5
Cleavage : Three; highly perfect and easy at 90 degrees.
Luster : Metallic
Color : Lead-gray
Streak : Lead-gray
Density : 7.58
Diagnostic Features and Trivia : The metallic luster, cleavage, and high density of galena, due to its lead content, makes it relatively easy to identify.
Crystal System and Habit : Isometric; common cubic crystals or cubo-octahedral
Twinning : Twin plane {111} penetration or contact; Twin plane {114} lamellae creating diagonal striations on cleavage faces.
Polymorphs : None
Index of Refraction : Opaque