Opal. Close-up of the cut and polished surface of an opal, a type of precious stone. Opal is a form of hydrated, amorphous silica. The glittering iridescent colours of an opal are due to water molecules trapped in the silica structure. This is a Yawah Nut opal, in which the opal (blue) is contained within concentric rings of limonite. It was found in Australia. Magnification: x3 at 4x5 inch size.
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