The nanostructure of fire opal

Fire opal, a transparent orange variety of opal, which does not diffract visible light, is built from the random accumulation of granular particles of hydrated silica, about 20 nm in size. This opal variety does not present the structure most commonly associated with precious opal, that is, a regular three-dimensional network of amorphous silica spheres about 200 nm in diameter. About 60 samples (from Mexico, Brazil, Kazakhstan, Ethiopia, Tanzania, Slovakia and USA) were documented using scanning electron and atomic force microscopy. This work demonstrates that nanograins are the elementary building blocks of this variety of opal.