How Do Geodes Get Their Colors?

Geodes are stones with a secret–at first glance, they seem like nothing more than the grey rocks you might see on walk around your neighborhood. Then–crack!–once they’re broken open, they reveal their inner beauty: a tiny cave filled with some of the world’s most spectacular, colorful crystals.

Born out of lava

Geodes are formed when there are pockets of air within rocks. This often happens after volcanic eruptions when lava cools around air bubbles. These pockets leave space for groundwater to seep in. But the water itself doesn’t produce geodes–it brings along minerals which stay in the rock even after the water evaporates. The minerals then start to build on each other to form crystals. It can take thousands or even millions of years for these crystals to form. The larger the crystals are, the older the geode is.

So what gives them their color?

The same minerals that form crystals can give them their glorious colors. Additional elements can also make their way into the mix and provide their own unique shades. Iron will give crystals a red or purple color, titanium will create blue, nickel or chromium leads to green, and manganese produces pink crystals.

While geodes can be naturally colorful some are artificially dyed. These dyed stones often have a brighter, more intense color than what appears naturally. Why do people dye geodes? Colorful geodes tend to sell well and can be a cheap way to imitate rare stones.

Come to the museum and check out the geodes of various colors on display in Hillman Hall of Minerals and Gems.

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