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| Earth on Oct 01, 2009

How are geodes formed?

Both sedimentary and volcanic geodes need just the right combination of water, chemicals, and pressure to produce the crystals inside.

You see geodes displayed in gift shops, sliced down the middle like a melon. They’re dull on the outside, but hollow and glittering with crystals on the inside.

Volcanic geodes are formed by cooling lava. Sedimentary geodes are formed underwater, possibly when sediments collect around the lifeless body of a sea animal. When the animal decays, it creates a cavity.

Both sedimentary and volcanic geodes need just the right combination of water, chemicals, and pressure to produce the crystals inside.

One way to open a geode – to reveal the crystals – is with a chisel and hammer. Score the geode completely around the outside where you want it to crack – usually in two equal halves. Keep going until it cracks and breaks apart. This will almost always work and won’t damage the crystals.

Trickier than opening a geode is recognizing one outdoors. Uncut, they look like your average rock. It helps to know where to look. You want a geodiferous outcrop of rocks. With practice, you can learn to identify a geode. They’re usually round or egg-shaped, and the weathered ones look like cauliflower on the outside.