Empressite is a mineral

Mineral

A mineral is a naturally occurring solid chemical substance formed through biogeochemical processes, having characteristic chemical composition, highly ordered atomic structure, and specific physical properties. By comparison, a rock is an aggregate of minerals and/or mineraloids and does not...

 form of silver telluride

Silver telluride

Silver telluride is a chemical compound, a telluride of silver, also known as disilver telluride or silver telluride. It forms a monoclinic crystal...

, AgTe.
It is a rare, grey, orthorhombic mineral with which can form compact masses, rarely as bipyrimidal crystals.

Recent crystallographic analysis has confirmed that empressite is a distinct mineral with orthorhombic crystal structure, different from the hexagonal Ag_5-xTe₃ with which empressite has been commonly confused in mineralogy literature.
At the same time, empressite does not appear on the equilibrium Ag-Te phase diagram, and therefore it is only metastable at ambient conditions. Given infinite time, it would phase separate into pure Ag₅Te₃ and pure Te.

The name empressite comes from the location of its discovery – the Empress Josephine mine, Saguache County, Colorado

Saguache County, Colorado

Saguache County is the seventh most extensive of the 64 counties of the state of Colorado of the United States. The county name comes from a Ute language word meaning “blue earth” or “water at blue earth”. The county population was 5,917 at U.S. Census 2000...

, USA. It was first described in 1914.