Great South Gems & Minerals

• Geodes, Chocolate, Whole •
(Mexico)

CLICK IMAGE TO ENLARGE

• Geodes, Chocolate, Whole •
(Mexico)

Geodes - rounded nodule - polished geodes

The Audubon Society Field Guide to North American Rocks and Minerals describes a "geode" as a rounded nodule, lined with crystals, usually quartz or calcite. I have always thought of a geode as being a rounded rock with a cavity in the center. If the stone does not have a cavity in the center it is referred to as a nodule. If it is hollow, it's a geode.

On a few occasions I have broken a geode open and found that the inside cavity was filled with water. Of course I didn't know that at the time and had no way of knowing that it had water in it. One of the guys here asked "How old is that water?" I I told them "As old as the rock!"

Geodes can form in any cavity, but the term is usually reserved for more or less rounded formations in igneous and sedimentary rocks, while the more general term "vug" is applied to cavities in fissures and veins. They can form in gas bubbles in igneous rocks, such as vesicles in basaltic lavas, or as in the American Midwest, rounded cavities in sedimentary formations. After rock surrounding the cavity hardens, dissolved silicates and/or carbonates are deposited on the inside surface. Over time, this slow feed of mineral constituents from groundwater or hydrothermal solutions allows crystals to form inside the hollow chamber. Bedrock containing geodes eventually weathers and decomposes, leaving them present at the surface if they are composed of resistant material such as quartz.

See cut & polished geodes
See a more complete explanation of how geodes are formed
Read article about Cracking Geodes open (without destroying them!).

(You can always remove items from your cart later if you wish)