Special Colors and Optical Effects

ABSTRACT

Oregon sunstone is a unique feldspar gemstone that displays a wide range of colors and optical effects due to the presence of metallic copper inclusions of various sizes and shapes. Most striking are the distinct green and red colors caused by copper particles, sometimes with strong red to green pleochroism. Recent computation work suggests that copper particles can strongly absorb and/or scatter light depending on the relative orientations of the feldspar crystal, 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. Common optical properties of Oregon sunstone are described and explained in detail in this paper, based on the optical theories of metallic nanoparticles in anisotropic media. Notably, it was found that the red to green colors observed can only be explained by considering both selective absorption and scattering from copper particles. The absorption and extinction spectra of the copper inclusions in Oregon sunstone are measured for different crystal orientations and polarizations. The scattering effect of the copper particles is quantified by subtracting absorption from extinction, the result of which agrees closely with the computed optical properties of spheroidal copper nanoparticles. The orientation of the anisotropic copper particles relative to the optical indicatrix of the feldspar is described. The correlation between the copper concentration profile and the “watermelon” color zoning in some Oregon sunstones is examined and discussed, along with a comparison to the color zones in treated, copper-diffused feldspar. Further studies on the diffusion and dissolution mechanisms of copper in feldspar crystals are required to fully understand the geologic processes that create these special gemstones in nature, which may provide additional methods to differentiate between treated and untreated stones.