How is a geode or an amethyst crystal rock formed ?
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Magma and lava is bubbly slush with all kinds of minerals in it. As the lava cools it turns into rock (basalt), and that rock can have bubbles in it. Those bubbles make empty space for the amethyst to form.
Enter water. Water is a solvent which means that minerals can dissolve in it. The minerals are still there, but they flow with the water (think of when you put salt in water). As the water moves through the porous lava-formed rock it picks up some minerals and carries them with. But sometimes the minerals fall out of the water (we call this precipitation).
If the minerals are right (iron and silica), the deposits form crystals inside the bubble.
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