Geodes are rocks that look like ordinary rocks on the outside, but are hollow and lined with crystals on the inside. Geodes are found worldwide, but are most concentrated in deserts. Geodes are usually round, sometimes with cauliflower-like bumps on the exterior. They must be broken or cut to reveal the crystals. The way geodes are formed isn't known for sure. They may be formed differently in different circumstances and in different locations. Three main types of minerals form most geodes.
Most geode crystals are quartz. The quartz is a silicate, which is the most common mineral in the earth's crust. More than 90 percent of minerals in the earth's crust are silicates. If another mineral, such as magnesium, is present when the geode is formed, it adds a different color to the crystal. Magnesium in quartz creates amethyst geodes. Heat can change the color of the geode. If an amethyst geode is exposed to a lot of heat it becomes a soft yellow, or citrine.
Calcite's main ingredient is calcium carbonate, which is made up of calcium, carbon and some of the oxygen left over from the water that left the crystals behind. Most calcite is white, though some of it is fluorescent. Like with quartz, adding another type of mineral changes the appearance of the crystal. Magnesium in calcite creates a pink dolomite layer.
Celestite or celestine is made of strontium sulfate. Celestite geodes are white or colorless at the base, with tips that are blue. The name "celestite" is from the Latin for sky. These crystals are brittle and don't work well for jewelry. Celestite gets its blue color from strontium.