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Smithsonian National Museum of Natural History

Opal replacing wood

Photograph of fossilized wood replaced by opal (84401)
Photo by Chip Clark. Click to zoom.
Catalog Number NMNH 84401-00
Locality Idaho, United States

From the exhibit

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.

Take a look at:

  • black opals, in which colors flash against a dark background.
  • white opals, in which colors play against a white background.
  • opals with no play of colors.

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