Millerite's environments of formation, mainly in hydrothermal replacement deposits and in limestone and dolomite cavities and associated geodes, usually serve as the best way to distinguish it from the aforementioned minerals. It is a real bonus to quartz geode collectors from Indiana to Kansas to open up a geode and find a spray of brassy millerite crystals tucked inside. It is postulated that the source of the nickel for the millerite that is found in these geodes is somehow derived from a meteoritic origin. Perhaps nickel, dissolved from iron-nickel meteorites, leached into the geodes by way of ground water and found an environment suitable for the formation of millerite crystals. When found as brassy sprays inside of sparkling clear quartz geodes, millerite can make a wonderfully attractive and interesting mineral.
Color is brassy yellow .
Luster is metallic. Transparency Crystals are opaque. The aggregates form radial sprays or randomly dispersed individual hairs. Also seen as fibrous coatings and rarely found in granular masses. Millerite is one of several minerals that is found in iron-nickel meteorites .