Nickeline (a.k.a. niccoline, niccolite) is a nickel arsenide mineral - NiAs. It has a metallic luster, a metallic pinkish-orange to pale copper color that tarnishes gray, and a gray streak. Nickeline is fairly heavy for its size, is moderately hard (H=5 to 5.5), and has no cleavage. It tends to occur with some Ni-Fe-Cu sulfide/arsenide minerals and with some Co-Ag minerals.
Nickeline (field of view ~1.7 cm across)