SPECIAL COLORS AND OPTICAL EFFECTS OF OREGON SUNSTONE

Oregon sunstone is a unique feldspar gemstone that displays a wide range of colors and optical effects due to the metallic copper inclusions of various sizes and shapes. The copper particles can strongly absorb and/or scatter light depending on the relative orientation among the feldspar crystal (optical indicatrix), the copper particles and the polarization of the incident light, resulting in dramatically different appearances of the same crystal when viewed (or cut) along different directions.

The absorption and scattering power of spheroidal metallic copper nanoparticles are calculated using the T-matrix method (simplified solution of Maxwell’s equation). The absorption peak wavelength shifts from ~560nm to over 600nm with the elongation of the particles, and the scattering power increases with increasing particle sizes. The experimental absorption and scattering spectra of both treated and natural copper-sunstones agree nicely with the computational results. The orientations of the copper particles and the optical indicatrix are not constrained by the triclinic symmetry of feldspar, which makes the light behavior inside an anisotropic Oregon sunstone crystal very complicated. However, by measuring the extinction spectra along special directions of an optically orientated crystal, the orientation of the copper particles can be accurately measured, which turned out to be along the a-axis of the feldspar crystal as expected. The copper concentration is strongly correlated with the color zoning in natural “watermelon” colored Oregon sunstones, whereas no obvious correlation between composition and color is observed in lab treated copper feldspars, which reflects the different timescale of the laboratory and natural processes producing the color in copper-bearing feldspars.