Schiller

A variable optical phenomenon in gems or minerals, related to sheen; varies from a milky to an almost metallic shimmer to a vivid iridescent play of color. It is usually seen just below the surface in certain varieties of pyroxene, feldspar, etc. Schiller effects are the result of thin microscopic inclusions within a translucent mineral, usually as exsolution lamellae, which refract and reflect incident light. Labradorite, a Na-rich anorthite (a plagioclase feldspar), sometimes exhibits a very pronounced iridescent schiller, also termed labradorescence, in particular where it contains appreciable K. This is due to the presence of a miscibility gap, causing exsolution of orthoclase from the plagioclase host during cooling. Gemstones cut from this material have been termed spectrolite. K-feldspars (probably between microcline and orthoclase in structure) with well developed schiller due to microperthite (exsolution of albite), produce semi-precious gemstones usually known as moonstones. Adularia, a low temperature K-feldspar of variable structure, also can exhibit this milky schiller, also termed adularescence. Sunstone or aventurine feldspar is a variety of feldspar (microcline or oligoclase) that has schiller with an orange/brown background color, and containing small hematite crystals that give it an additional sparkle.