Geodes are full of surprises and you don't truly know what they will look like until you crack one open. How do geodes form? Before we answer this question, first explain what a geode is. A geode is a rock which contains a cavity of empty space on the inside. If a massive rock does not contain any empty space it is not a geode, this cavity is typically lined with crystals such as quartz, calcite or agate. And must note that not all geodes are round in fact many geodes exist that have irregular shapes.
The first step in the formation of a geode is a volcanic eruption which occurs near or above the existing crust; a lava flow needs to be produced as not all volcanic eruptions emit lava. As the lava flows out from the vent it mixes with gases such as carbon dioxide and sulfur dioxide. Most of these gases escape the erupting material on occasion these gases can get trapped inside the lava. This forms large bubbles within the material. Volcanic gases are not the only thing that gets trapped in certain circumstances such as undersea eruptions or when lava hits a groundwater aquifer large amounts of water can also get trapped within the lava. As the lava cools these gases and water get trapped in a layer of recently called stone formed from the eruption. Whatever shape these bubbles were at this stage will determine the resulting shape of the geode. Due to the chemicals present within the rock crystals begin to grow at this stage the longer the crystals take to cool the larger they become. As these crystals grow the gas and water become compressed until it finds a weak point in the rock and escapes. After the water and gas are gone the hollow area remains, but it is now partially filled with beautiful crystals.
Around the world there are hundreds if not thousands of geo beds; these geodes typically form in basaltic or rhyolitic volcanic rock of all of these aforementioned beds; the most famous one is in Uruguay. This geode bed has produced most of the world's largest geodes.