Freibergite - argentiferous variety of Tetrahedrite.
  
From: Silvergruvan, Wermland, Sweden.
  
Massive Freibergite disseminated in Quartz and some Chalcopyrite and Galena . Silvery gray in color and metallic luster. Sample measuring 2.5 x 2.5 x 2.0cm. Associated with silver-rich sulphide and sulfosalt veins.