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Gem opal

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On this page you can find :

perfect diffraction in opal,
rainbow diffraction

Perfect diffraction in an opal from Wollo, Ethiopia

This video shows a 4.43 ct oval cabochon of white and play-of-color opal from the Wollo Province, Ethiopia. The play-of-color does not form large patches of spectral colors, but a network of sharp points of spectral colors moving together as the white light source (an optical fiber) moves around the sample. Note that the color of each spot slightly changes with the position of the white ligth source. All this indicates that this sample is made of a single optical crystal, i.e. a single network of silica spheres running from side to side of the sample, over one centimeter. This also explains that the diffraction produces sharp points and not large areas. This is absolutely exceptionnal.
From a crystallographical standpoint, this feature is like a Lauegram, first described by Max von Laue for the diffraction of X-rays by a (true) crystal. As opal diffracts visible light, such sample allows the visualization of the reciprocal lattice responsible for the diffraction of light. The reciprocal lattice is the Fourier transform of the "crystal lattice", i.e. the direct lattice of the silica spheres network. If you wish to learn more on the crystallography of opals, follow the DUG program!

This extraordinary phenomenon indicates a perfectly continuous network of silica spheres over more than one centimeter in all directions. Hence, the conditions of formation of such an opal must have been remarkably quiet, with no perturbation during the "crystallization" of this opal.

Rainbow opal

Some rare samples of opal from Wollo, Ethiopia, show this nice optic effect: diffraction colors do not form large patches of homogeneous color, but all colors of the rainbow are observed in small areas. these rainbows are best observed in there whole under a diffused white light. This video shows the phenomenon under a fiber optic, in order to examine how color change while the source is moved aroud the sample.
The exact reason for these rainbows to appear remains unknown, but it could result from the diffraction on a curved network or on a striated surface.

Site web par . Auteurs : - Institut des Matériaux, 2 rue de la Houssinière, B.P. 32229, 44300 Nantes, France, et Benjamin Rondeau.