geodeserna

Geodes are spherical to oblong rocks that typically contain a hollow cavity lined with crystals. They form when mineral-rich fluids or gases create a void in preexisting rock—such as a gas bubble in lava or a void in sedimentary rock—and later precipitate minerals on the cavity walls, gradually building crystal formations.

The interior crystals can include quartz, amethyst, calcite, celestite, agate, gypsum, and other minerals. Crystal size and color vary with mineralogy and trace elements; amethyst geodes are particularly valued for their purple crystals. The outer surface is usually rough and earthy, often with a contrasting rind that marks a hard outer shell.

Geodes are found worldwide. Notable sources include basaltic regions in Brazil and Uruguay, and sedimentary deposits in the United States (notably Iowa, Kentucky, and Missouri), as well as Mexico, Spain, Morocco, and China. They are commonly collected by hobbyists and sold as specimens; when opened or cut, they reveal the crystal-lined interiors. Geodes are also used for decorative purposes and in lapidary work.

Geodes differ from nodules and vugs. A geode is a rock containing a hollow cavity lined with crystals, whereas nodules are dense, rounded masses without a sizable hollow, and vugs are openings within rocks that may or may not be lined with crystals.