This Is How Geodes Get Their Colors

The Carnegie Museum of Natural History gives us a brief version of how geodes and their colors form. Like many other geological phenomena, it all starts with volcanoes. When gases build up under Earth's surface, volcanoes erupt and spew out magma, gas, and ash. Lava is the term given to magma once it's on the surface and flowing, bubbling, roiling, etc. Lava is super-hot, molten, or semi-molten rock that is at least 1,300 degrees Fahrenheit and can reach 2,200 degrees. It's made of numerous elements, per USA Today, including oxygen, aluminum, calcium, silicon, iron, titanium, etc. There are also different kinds of lava based on temperature and composition.

But no matter how hot, lava cools. When it cools it hardens into a bumpy, rocky-looking surface. The surface isn't perfectly sealed, however, and when it rains water seeps inside. Rainwater carries minerals along with it as it flows, and when the water evaporates it leaves those minerals behind. Get enough of those minerals built up and viola: thousands or millions of years later, you have a geode. 

As Michel and Company explain, the type of mineral carried along and left behind determines the color of geode. If there's a lot of magnesium and manganese, a crystal will look pink. Iron creates a "rusty red" color or purple, while titanium makes things blue. Depending on what it's mixed with, copper can produce red, green, or blue crystals. Nickel, meanwhile, makes crystals green. And if a crystal gets super hot it turns yellow-orange.

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