IRIDESCENT GEM MATERIALS AT THE NANOSCALE (Invited Presentation)

Most colored gemstones acquire their chromatic hues through the selective absorption of visible light via a range of mechanisms that include band gaps (blue and yellow diamonds), crystal field stabilization energies of transition metals (hematite, ruby), and F-centers (fluorite). In contrast, an exclusive class of gem materials exhibits structural coloration, which arises from the constructive interference of light waves during interactions with mineral nanoassemblies. Structural color can impart chromatic characteristics that cannot be matched by pigments: 1) Structural color can alter dramatically with viewing angle; 2) The human eye typically perceives structural coloration as more vivid than pigmented color; 3) Structural color is lightfast whereas pigmented color tends to fade upon prolonged exposure to ultraviolet radiation. Consequently, structural coloration can elevate common and otherwise nondescript geological materials to expensive and highly coveted gems.

The presence of structural color can yield insights into crystal growth mechanisms. In contrast to biomimetic patterning, the simpler pathways by which minerals form offer a likelier stimulus towards technologies based on self-assembly of organized nanostructures. This talk will extend the exploration of structural color in minerals beyond the conventional rainbow gems (opal, labradorite, nacre) to less-studied iridescent minerals that have nevertheless proved enticing to the jewelry and collecting communities – hematite, goethite (turgite), rock crystal quartz, and iris agate. Using scanning and transmission electron microscopy in tandem with focused ion beam imaging and extraction, we have revealed surprising nanoscale motifs that give rise to constructive light interference, such as 3D photonic arrays of hematite nanoneedles, quasi-periodic void layers (turgite), blazed gratings of emergent twins (iridescent rock crystal), and hierarchical oscillations in twin defect density (iris agate).