Volcanic geodes are hollow rocks which contain a lining of crystals inside. They are formed in the cooling of lava, when pockets of air gets trapped inside, leaving a bubble in the molten lava. This ‘bubble’ cools quickly, creating a slightly porous extrusive rock form. Overtime, rain water seeps through the out side of the rock and can end up on the hollow inside. The minerals in this water solidify into tiny crystals. This process goes on for thousands of years until found. When found, people crack them open to reveal the crystals inside.
The colour of the crystal depends on the minerals in the water at the time.