Naturally iridescent materials such as opal or a butterfly’s wing have long fascinated onlookers for their ability to produce spectacular rainbow colors. Now, researchers have uncovered the cause of the vibrant display in another naturally iridescent material, the iron oxide mineral known as rainbow hematite. Pennsylvania State University geochemist Peter J. Heaney and his student Xiayang Lin used a variety of imaging and spectroscopy techniques to investigate the chemical makeup and surface structure of a rainbow hematite sample from Brazil (Gems & Gemol. 2018, DOI: 10.5741/GEMS.54.1.28). They found that the mineral contains stacked sheets of spindle-shaped nanocrystals that are arranged at 120-degree angles. The nanocrystal array acts as a diffraction grating, splitting and scattering beams of light to produce the rainbow effect. The researchers also calculated a chemical formula (Fe1.81Al0.23P0.03O3) for the mineral, which contains aluminum and phosphorus impurities in addition to its major iron and oxygen components. The presence of those impurities within the crystal structure may have prompted the nanoparticles to grow into spindle shapes rather than into more symmetrical rhombohedrons, Heaney says. He thinks the findings could inspire new nanocrystal-based iridescent coatings.