Fluorescence Characteristics of Two Copper-Diffused Plagioclase Feldspars: Labradorite and Andesine

Natural-color labradorite sunstone from Oregon has been available for more than a century. In the first decade of the 2000s, a type of "red andesine feldspar" was unveiled at the Tucson gem shows, and its origin remains an open question. There is still no convenient method that can quickly and reliably determine whether a red andesine feldspar has been treated. For this study, both colorless labradorite and light yellow andesine feldspars were copper diffusion treated to modify their color to mimic natural labradorite sunstone. Systematic studies of the fluorescence characteristics of both untreated and treated labradorite and andesine feldspars as well as natural gem-quality sunstone from Oregon and Ethiopia were conducted. Under 320 nm excitation, strong fluorescence emission near 394 nm and weak fluorescence emission near 554 nm were observed in the spectra of copper-diffused red labradorite and andesine feldspar. For comparison, natural red and green labradorite sunstone material from Oregon and Ethiopia were tested, and these exhibited substantially weaker fluorescence emission around 394 nm, similar to the untreated nearly colorless labradorite and light yellow andesine feldspar samples. This fluorescence characteristic, due to the presence of Cu+ ions in the plagioclase crystal structure, can potentially help to identify copper diffusion treatment.