Geode a term from a Greek word meaning earth shaped are irregular roughly spherical rocks. They can be oblong or shaped like invertebrate fossils. Many of the geodes found in Michigan and Indiana show distinct signs of fossil replacement. In southern Indiana Harrodsburg and Salem areas the same conditions are found right beside geodes that are not fossil replacements. Such other locations for fossil replacement geodes are Georgetown Ohio where gastropods are filled with a whole assemblage of different minerals Rucks Pit Fort Drum Florida is another fossil replacement geode location here Mercinaria clams are filled with very large golden calcite crystals. To further add to this growing list of fossil replacement geodes we cannot forget the coral replacement geodes of the Withlacoochee river near the Florida and Georgia boarder.
Some geodes are hollow and lined with the most beautiful and unusual layers and clusters of various minerals but others are completely filled by inward growing crystals no longer technically geodes but nodules or thundereggs. Hollow geodes relatively lightweight compared to those that are completely filled are more desirable because they generally contain a greater variety of minerals that have grown well formed crystals. I have found the following minerals myself in midwest geodes quartz calcite sphalerite dolomite aragonite ankerite barite pyrite smythite millerite marcasite kaolinite goethite limonite hematite and gypsum var selenite. When purchasing geodes the rarer minerals for sedimentary geodes are sphalerite aragonite smythite super rare and millerite. These are the most sought after minerals found in sedimentary geodes of the midwest.
Sedimentary geodes from Keokuk Iowa area have an outer shell of chalcedony. Once the outer shell forms mineral rich water still inside the shell may cause more quartz to be deposited and other minerals to form toward the center. Chalcedony much harder than the host rock of limestone helps to preserve the geode during weathering. As the weaker host rock is eroded the geodes weather out and remain behind for the collecting. The enviroment inside the chalcedony shell is an exellent place for crystal growth. As temperature and pressure change as well as evaporation causes the mineral matter to precipitate into the crystals we find and buy today. More mineral rich solutions may seep into the geode later adding to the quartz or forming other minerals this is called a secondary or even third mineralization and is not all that uncommon.
Some lava basalts have round or almond shaped gas holes partially or completely filled with calcite chalcedony agate crystalized colorless quartz amethyst or other minerals. Ground water carrying dissolved silica seep into the lava and basalt cracks and cavities depositing silica minerals.
The Hauser geode beds of California are found in groups located in volcanic ash beds. Mexican geodes found at Las Choyas and Trancas are also found in volcanic areas. There are 17 different rare magnesium minerals found in these geodes. Rare minerals found in volcanic geodes are the magnesium oxides selenite amethyst and calcite.
Larger geodes of both sedimentary and volcanic geodes are considered rare because it take special conditions for larger geodes to form as compared to a normal sized geode. Large geodes for sedimentary geodes would be any geode over the size of 6 to 8 inches. Volcanic geodes are considered large after 4 to 5 inches. The hollowness of a geode also contributes to the value. The more hollow the geode the bigger the crystals and more minerals that can be found inside. Agate shells in each catagory can make the geode more valuable as well.