Millerite is an odd, scarce nickel sulfide mineral (NiS).  It tends to form radiating clusters or tufts of long, hairlike needles.  Some geodes in the Mississippian limestone outcrop belts of America's Upper Mississippi Valley & Ohio Valley have millerite inside (see below).  Millerite crystals are very slender hexagonal needles with a metallic luster and a brassy-gold color.


Millerite (above & below) - spray of millerite needles in chalcedony-lined quartzose geode, from the Mississippian of Monroe County, Indiana, USA (CMC public display, Cincinnati Museum Center, Cincinnati, Ohio, USA). 



Photo gallery of millerite



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