Iowa State Rock

Geode

Adopted in 1967.

Because Iowa is well known for the presence of the Geode, it was chosen as the official rock in 1967 as an effort to promote tourism in the state. Legislators who favored making the Geode the state rock pointed out that it is among the rarest and most beautiful of rocks and that Iowa is known worldwide because of the large number found in the state. Other rocks considered for official status were limestone and fossil coral.

In Latin, the word "geodes" means "earthlike." Geodes are shaped like the earth and average about four inches in diameter. Geodes are found in limestone formations and have a hard outer shell. When carefully broken open, a sparkling lining of mineral crystals, most often quartz and calcite, is revealed.

Geodes have drab, rounded exteriors with a hard outer layer and partially hollow cavity lined with crystals. Some geodes are completely filled with small compact crystal formations such as agate, jasper or chalcedony and are called nodules. The only difference between a geode and a nodule is that a geode has a hollow cavity, and a nodule is solid.

Geodes begin as bubbles in volcanic rock or as animal burrows, balls of mud or tree roots in sedimentary rock. Over a period of millions of years, layers of silica cool, forming crystals of different minerals within the cavity. Different types of silica cool at varying temperatures, thus creating layers of different types of mineral crystals. The silica precipitation can contain many varieties of dissolved minerals, the most common being quartz, but calcite, amethyst and many others are also found.

Why Are Keokuk Geodes Special?

Keokuk Geodes are famous for their sparkling interiors containing some of the most beautiful crystals and minerals in the entire world

Quartz is the dominant mineral found in Keokuk Geodes and shimmering white to transparent quartz crystals fill the walls of many of these geode cavities but 19 additional minerals have also been identified including:

• Aragonite (occurs as groups of colorless to white crystals, very rare.)
• Barite (Crystals range from colorless to white, yellow, pink and pale blue.)
• Calcite (occurs as common crystals, dogtooth and nailhead types with colors white, brown and rarely pink.)
• Chalcedony (a microcrystalline mineral ranging in color from gray, black, blue, orange, red and the rare green. Chalcedony also makes up the geodes outer shell.)
• Chlorite (very rare black to gray coatings on crystals.)
• Chalcopyrite (crystals range from bright gold to bronze.)
• Dolomite (pink, brown or buff saddle shaped crystals sometimes dominate the entire geode cavity.)
• Goethite (occurs as black shiny elongated crystals.)
• Gypsum (crystals are long and clear or colorless also appears as flakes or grains.)
• Hematite (rare but occurs as tiny red crystals.)
• Jarosite (occurs as yellow to brown granular masses usually coating pyrite or marcasite, sometimes confused as sulpher.)
• Kaolinite (a bright white to brown clay mineral which usually fills the entire geode cavity.)
• Malachite (green inclusions and microscopic crystals.)
• Marcasite (occurs as masses of small crystals with a brassy or bronze color sometimes with a greenish tint.)
• Pyrite (occurs as reddish, gold or black cubic or capillary crystals.)
• Pyrolusite (usually appears as black stains on quartz.)
• Quartz (occurs as drusy crystals or large defined crystals ranging in color from clear, green, smoky, pink and the famous and rare citrine/orange.)
• Siderite (occurs as thin coatings that give a gorgeous iridescence to brown calcite.)
• Smithsonite (rare but appears as granular gray, blue, yellow or orange masses.)
• Sphalerite (rare but occurs as shiny black crystals often only one single large crystal.)

Southeastern Iowa is one of the state's best Geode collecting areas. Geode State Park, in Henry County, is named for the occurrence of the Geode.

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