Geodes are hollow rocks that contain crystals. They form when gas bubbles in lava or rock become lined with minerals. As the minerals crystallize, they grow inward, creating a hollow cavity that can be completely or partially filled with crystals.

Geodes can be found in a variety of colors, including white, brown, red, orange, yellow, green, and blue. The crystals inside geodes can also be a variety of colors, including clear, white, brown, red, orange, yellow, green, blue, and purple.

Geodes are found in many parts of the world, including the United States, Mexico, Brazil, Uruguay, and Madagascar. They are typically found in volcanic rocks, such as basalt and andesite.

Geodes are popular collector's items. They are also used in jewelry, art, and home décor.