Intro to Amethyst Geodes
Not even scientists are 100 percent sure exactly how amethyst geodes form--or how any geodes form. There hasn't been much research, because geodes are just fun scientific anomalies with little, if any, scientific benefits. They're rocks that seem plain on the outside but when opened reveal a cavity in the middle filled with beautiful crystals. The general scientific consensus is that amethyst geodes are created in a two-step process. First, there's the formation of the cavity and then the formation of the crystals.
Gas Cavities Form
An amethyst geode is a hollow rock with amethyst crystals lining the inner walls, so first the cavity must be formed. This can happen anywhere there is or was lava close to the earth's surface. As a result, amethyst geodes can be found in thousands of places around the globe. The first step in the natural process that creates amethyst geodes is the formation of gas cavities in lava. The gas cavities can form from bubbles (just like the carbonation causes bubbles in your soda). Some scientists theorize that the cavities can also be formed when cooling lava flows near tree roots or other things sticking out of the ground. The cooling lava hardens completely before filling in around the outcrop, creating a cavity.
The Cavities are Filled
The cavities then fill up with a silica-rich liquid that contains trace amounts of iron. Over time, this liquid forms crystals -- six-sided pyramids (rhombohedrons) of amethyst. Crystals with a color that range from light lilac to deep purple are formed when there is a trace of iron in the liquid, resulting in amethyst geodes.