The Origin of Color in ‘Fire’ Obsidian

Chi Ma and George R. Rossman
Division of Geological and Planetary Sciences, California Institute of Technology,
Pasadena, CA 91125, U.S.A.

James A. Miller
GeoEngineers, Inc., Redmond, WA 98052, U.S.A.

Fire Obsidian


A variety of obsidian from Glass Buttes, Oregon, named for thin layers showing various colors and known as ‘fire’ obsidian, was investigated with FE-SEM, EDS, EBSD, and optical spectroscopy methods. Our study reveals that the thin layers consist of mainly concentrated nano-sized magnetite crystals. The thin layers having a thickness of 300 nm to 700 nm give rise to brilliant colors in reflection. The color is caused by thin-film interference where the thin layers have a higher calculated refractive index (n=1.495 ~1.512) than that of the host glass (n=1.484).

 Fire obsidian in the host rock.

Fig 1a    Fig 1b   samples 

Fig 2    in host rock

Figs 3abcd 

Fig 4 optical spectrum  Fig 4 (large version) 

Fig 5a   Fig 5b   Fig 5c   Fig 5d (tif files - use 'save as')    SEM images

Fig 6a (bmp file) Fig 6b (jpg file) EBSD patterns

Canadian Mineralogist 45, 551-557