Gersdorffite is a nickel mineral with formula NiAsS. It crystallizes in the isometric system showing diploidal symmetry. It occurs as euhedral to massive opaque, metallic grey-black to silver white forms. Gersdorffite belongs to a solid solution series with cobaltite freely substitutes also leading to ullmannite , NiSbS. It has a Mohs hardness of 5.9 to 6.33.
Gersdorffite has three crystallisation forms: Gersdorffite-P213 (NiAsS), Gersdorffite-Pa3 (Ni(As,S)2) and Gersdorffite-Pca21 (NiAsS). Gersdorffite occurs as a hydrothermal mineral along with other nickel sulfides. Associated minerals include nickeline , nickel-skutterudite , cobaltite, ullmannite, maucherite, platinum-group minerals, millerite, and chalcopyrite.
Gersdorffite was first described in 1843 and named for Johann von Gersdorff (1781–1849), owner of the nickel mine at Schladming, Austria the type locality.