What do opals and Yellowstone, the world’s most famous supervolcano and national park, have in common?

Opal, a hydrated silica, is closely related to geyserite, that fantastically-shaped rock you see around hot springs and geysers.

Indeed, opal is the only precious gemstone that isn’t a mineral.

Common opals are milky white or bluish, but fire burns inside precious opals. According to the Gemological Institute of America, this fiery play of color:

…occurs in precious opal because it’s made up of sub-microscopic spheres stacked in a grid-like pattern—like layers of Ping-Pong balls in a box. As the lightwaves travel between the spheres, the waves diffract, or bend. As they bend, they break up into the colors of the rainbow, called spectral colors. Play-of-color is the result.

So pretty!


  1. Common opals can be waxy, yellow, brown, red, grey, pale green… Nickel opals are bright apple green. And many opals include black dendrites of manganese oxides.

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