The Collaborative International Dictionary of English v.0.48:

Nagyagite \Nag"yag*ite\, n. [So called from Nagyag, in
   Transylvania.] (Min.)
   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.
   [1913 Webster]