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Why does old glass in the desert take on iridescent colors?

Answer:

It is sometimes called solarized glass. Glass is made from quartz sand, and most deposits of sand contain some iron. That gives the glass a green or brown color. Manganese dioxide is added to the glass to oxidize the iron, preventing it from coloring the glass. Long exposure to ultraviolet rays can cause the manganese to convert back into its oxidized form. The iron color does not come back, but the oxidized manganese colors the glass various shades of pink, purple, and blue. Other colors can sometimes be found. If the glass was decolorized with selenium or cerium, the solarized glass may be yellow or brown.
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