Sedimentary Rocks.docx

Sedimentary Geodes Geodes in sedimentary rocks are usually found in limestones, dolomites, and calcareous shale. In these deposits a gas-filled void can serve as the opening for geode formation. Shells, tree branches, roots and other organic materials often decay away to leave a void for the formation of mineral materials. These cavities can be filled with quartz, opal, agate or carbonate minerals. They are generally smaller than the geodes formed in volcanic rocks. Geodes are most easily collected when their host rocks have weathered away. This can occur because basalt, limestones, dolomites, and shales weather much more readily and rapidly than the quartz and chalcedony that typically form the outer layer of a geode. The host rock weathers away and the geodes are left on the surface, washed into a stream, or stranded in a residual soil. In these situations the geodes are easily found and collected. Some geodes are produced by mining the host rock, but that method is difficult, costly, and often damages the geode.

Naming Geodes Geodes are given a variety of names. The word “geode” is often preceded by the name of the mineral material which has filled the geode. “Agate geode” and “amethyst geode” are examples. The word “geode” might also be preceded by a geographic or stratigraphic name. “Keokuk geode” and “Brazilian geode” are examples.

Geodes ready for shipment: Geodes from Brazil and other collecting localities are carefully crated to prevent damage during shipment. This photo shows a matched pair of cathedral geodes which are two halves of the same cavity. They have been given bases of heavy concrete that will enable them to be used as items of decor in a home or office. Image copyright iStockphoto / VYG.

Dyed Geode Slices These dyed geode slices have been mounted in wooden bases for display. Image copyright iStockphoto / danishkhan.

Amethyst geode sections: Small sections of amethyst geodes offered for sale in a tourist shop. Image copyright iStockphoto / danishkhan.

Small Geodes: Small geodes like these are often sold in novelty and science stores to people who want to have the experience of opening a geode themselves. They want to be the first human to see the treasure inside. Image copyright iStockphoto / leopardhead.

Commercialization of Geodes An unopened geode has the appearance of an uninteresting rock. They become much more interesting when they are opened and their internal crystals and agate bands become visible. And even though every crystal-lined geode is a wonder of nature, there are many things that can be done to make it into a more marketable product as well as enhance its value.

Museum-Quality Geodes But, before we go any farther, we want anyone who finds a really nice geode to know that museum-quality geodes are sold in the collector market for prices that will astound the average person. The Arkenstone, a business that specializes in museum-quality mineral specimens, usually has a few nice geodes for sale. At the time this article was being written, they were selling rare-mineral lined geodes featuring volkovskite, millerite, thomsonite, and shattuckite for four- and five-digit prices. You can see some of their inventory here.