Millerite is one of a few sulfide minerals that form fine acicular crystals that appear as hair-like fibers aggregated into sprays. Millerite is also called "Capillary Pyrite" since it has a brassy yellow color that is close to the color of pyrite and forms the trademark capillary crystals. When found as brassy sprays inside of sparkling clear quartz geodes, millerite can make a wonderfully attractive and interesting mineral.