Opals

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Opals are naturally occuring 3D photonic bandgap materials. They consist of a lattice of silica spheres arranged in a way so that their spacing is roughly the same as the wavelength of light. The different colors seen in an opal are due to different areas having slightly different spacings, and so some regions will strongly reflect red ligh, others blue, others green etc. Thus, opals can be thought of as microcrystaline materials. You can read more about them in R. Shankars final report.FinalWikiRshankar