Special Colors and Optical Effects of Oregon Sunstone: Absorption, Scattering, Pleochroism, and Color Zoning

Abstract

regon sunstone is a significant feldspar gemstone with growing popularity, and certainly one of the most famous gemstones produced in the United States. Found in weathered lava flows, these basaltic phenocrysts have poorly developed cleavages and few inclusions or twins due to the disordered structure resulting from a fast cooling rate (Xu et al., 2017;Jin et al., 2018Jin et al., , 2019, making them more suitable for faceting than other feldspar gemstones. Their popularity comes mainly from the wide range of appealing colors (figure 1), from the more common yellow, pink, orange, and red to the extremely rare green-blue (www.oregonsunstoneguide.com/typesand-colors.html). Many of these colors have not been found in feldspar from anywhere else in the world. The term sunstone was originally defined (Andersen, 1915) as the gem variety of aventurine feldspar, showing intense play of light and colors caused by strong reflections from thin oriented metallic inclusions of visible sizes, a phenomenon known as aventurescence. The loosely defined word schiller is more commonly used to describe the aventurescence of sunstones in the gem community (the two terms are used interchangeably in this article). Most sunstones, such as those from India, Norway, and Tanzania, owe their aventurescence effect to hematite inclusions. Copper-containing aventurine feldspar was first documented by Andersen (1917), who studied specimens reportedly from Modoc County, California. This county borders southern Oregon, an area hosting