THE ORIGIN OF COLOR IN “FIRE ” OBSIDIAN

A variety of obsidian from Glass Buttes, Oregon, known as “fi re ” obsidian and named for thin layers showing various colors, was investigated with fi eld-emission scanning electron microscopy, X-ray energy-dispersive spectroscopy, electron back-scatter diffraction, and optical spectroscopy methods. Our study reveals that the thin layers mainly consist of concentrated nanometric crystals of magnetite. The thin layers, which have a thickness of 300 to 700 nm, give rise to brilliant colors in refl ection. The color is caused by thin-fi lm optical interference, in which the thin layers have a higher calculated index of refraction (1.496 < n < 1.519) than that of the host glass (n = 1.481)