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!