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TITLE: Opals and Moonstones

[Photo: Opals and Moonstones]

Turn a precious opal this way and that, and watch the colors dance. Within the opal, tiny spheres of silica—stacked up like oranges—scatter light, causing the play of colors. Opals with this fiery flash are prized as gems. But water comprises 6-10 percent of their weight, and heat can dry them out and make them crack. Opal is the October birthstone. Look at these minerals from one direction, then another, to catch the play of light. Labradorites, moonstones, and sunstones belong to the feldspar family, which makes up over half of the Earth's crust. Occasionally, this common material forms crystals that shimmer like a rainbow on a soap bubble. This iridescence occurs when closely spaced layers in a crystal diffract light, separating it into its component colors.

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