Nagyagite

Nagyagite (Pb5Au(Te,Sb)4S5-8) is a dark grey to black sulfide. Nagyagite crystals are opaque and orthorhombic.
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(n.)
A mineral of blackish lead-gray color and metallic luster, generally of a foliated massive structure; foliated tellurium. It is a telluride of lead and gold.
  

General Information:
 Chemical Formula:
AuPb(Sb,Bi)Te2-3S6
 Composition:
(Molecular Weight = 1,059.12 gm)
Bismuth 4.93 % Bi
Antimony 8.62 % Sb
Tellurium 30.12 % Te
Lead 19.56 % Pb
Gold 18.60 % Au
Sulfur 18.17 % S 
 Empirical Formula:
AuPbSb0.75Bi0.25Te2.5S6
 Name Origin:
Named after the deposit Nagyag (now Sacaramb), Romania.
Physical Properties:
 Cleavage:
[010] Perfect
 Color:
lead gray or black.
 Density:
7.5
 Diaphaniety:
Opaque
 Habits:
Granular - Generally occurs as anhedral to subhedral crystals in matrix., Lamellar - Thin laminae producing a lamellar structure., Platy - Sheet forms (e.g. micas).
 Hardness:
1.5-2 - Talc-Gypsum
 Luminescence:
None.
 Luster:
Metallic
 Streak:
gray black
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