EarthSky // FAQs // Earth By EarthSky 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.

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9 Responses to How are geodes formed?

  1. yankes222 says:

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  2. ilovescience says:

    this is awesome

    • Vip says:

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  3. SLC Dentists says:

    That is really intriguing. It provided me a few ideas and I’ll be posting them on my website eventually. I’m bookmarking your site and I’ll be back. Thanks again!

  4. val says:

    Awesome-ness!! Thanks Earthsky!

  5. Vlad says:

    Dude, this is not toinformitave. it just gave me a vauge answer, I need more detail than this!

  6. lifesucks says:

    i really like this i am the only one in my grade that got an a on there science fair project

  7. Emmy says:

    we made these in science class out of eggs chemical and food coloring!! We cut the blew the inside of the egg out into a bowl then cut the egg in half. After that we painted them any color and sprinkled this chemical onto them! Over night they changed and most of them looked like real geodes!!
    It was a really cool expirment to try! :)

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