Iowa State Stone
A geode is a hollow rock that is partially or completely filled with crystals. The crystals are sometimes made of the minerals calcite, kaolinite, pyrite, or dolomite, but most are composed of varieties of quartz, including amethyst and rose quartz. Some even contain gem-quality opal. Chalcedony may also line the insides of geodes, covering crystals and providing colors such as white, gray, blue, yellow, and orange. Geodes may also contain agate or jasper banding, and some are completely filled with banded quartz. A layer of agate surrounds most geodes.
Crystals range in size from microscopic to quite large. Most geodes are two to six inches (7.6-15.2 cm) in diameter, but specimens of over three feet (1 m) wide have been found.
A STATE SYMBOL
Iowa is well known for the occurrence of geodes, especially in the southeastern part of the state. The collecting area comprising roughly a 35-mile radius of Keokuk, Iowa, is known worldwide among rock collectors. The Warsaw Formation in southeast Iowa and northeast Missouri is also important for the many geodes that formed there. Geode State Park in Henry County is named for these rocks. Most of Iowa’s geodes are filled with quartz crystals, and many of those crystals are of gem quality. As an attempt to stimulate tourism in the state, the Iowa General Assembly designated the geode the official state rock in 1967.
The Latin word geode means "earthlike," referring to its roughly spherical shape.
FORMATION AND OCCURRENCE
Geodes are round with hard outer shells. They occur in igneous and sedimentary, usually limestone, formations. The crystal growth in sedimentary rock is most likely due to groundwater circulating through the rock during its formation, which deposited minerals over millions of years.
Geodes may also form where internal fluid pressure causes rock cavities. In volcanic rock, groundwater carrying dissolved silica flows into gas holes in cooling lava. These minerals then crystallize, and a geode is formed. Because geodes have an outer layer that is more resistant to weathering than its host rock, they are commonly deposited onto stream bottoms.
Geodes can be numerous in the areas they occur, but these areas are limited.
Amateur and experienced rock collectors alike appreciate geodes. There is no way to know what the crystals inside a geode look like without cracking it open. This is usually done fairly easily with a rock hammer.
Although geodes are not used in jewelry, cabochon and tumbled gems can be made from thin linings of some geodes.
Chemical Formula: SiO2
Crystal Structure: Trigonal
Hardness (Mohs): 7; lower in impure varieties
Color: Clear (in pure form)
Birefringence: +0.009 (B-G interval)
Refractive Index: 1.544-1.553 - Dr +0.009 (B-G interval)
Density: 2.65 constant; variable in impure varieties
|Author: World Trade Press|