A white surface having iridescence. In opal gemstones this "play of color" is caused by tiny spherical formations of silicon inherent in this stone's structure. These spheres, and the spaces in between, refract white light much as a diffraction grating spectroscope, separating wavelengths of the various spectral colors. The colors seen depend on the size and distance between the spheres in the opal's structure.Example:Josef Hoffmann (Austrian, 1870-1956) for Wiener Werkst?tte, Square Brooch, silver lattice, repouss? gold, and opal, c. 1905. See Art Nouveau, jewelry, and secession.Also see fluorescence, fluorescent colors, glaze, glitter, and semi-gloss.