1) Iron mineral 
Berthierite is a mineral, a sulfide of iron and antimony with formula FeSb2S4. It is steel grey in colour with a metallic lustre which can be covered by an iridescent tarnish. Because of its appearance it is often mistaken for stibnite. It was discovered in France in 1827 and named for the French chemist, Pierre Berthier (1782–1861). ...
• (n.) A double sulphide of antimony and iron, of a dark steel-gray color.
Ber'thi·er·ite noun [ From Berthier , a French naturalist.] (Min.) A double sulphide of antimony and iron, of a dark steel-gray color. 
Berthierite, named after the French naturalist Berthier, is a double sulphide of antimony and iron, of a dark steel-grey colour.