Great South Gems & Minerals

• Geodes, Buchanan, Whole •
(Oregon)

CLICK IMAGE TO ENLARGE

• Geodes, Buchanan, Whole •
(Oregon)

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 are rock formations which occur in sedimentary and certain volcanic rocks. They are normally spherical in shape and has a cavity, or vug, in which internal crystal formations or concentric banding. The exterior of the most common geodes is generally limestone or a related rock, while the interior contains silica, or quartz crystals and/or chalcedony. Other geodes are completely filled with crystals, being solid all the way through. These types of geodes 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, tree roots or mud balls in sedimentary rock. Over time, the outer shell of the spherical shape hardens, and water containing silica precipitation forms on the inside walls of the hollow cavity within the geode. The silica precipitation can contain any variety of dissolved minerals, the most common being quartz, but amethyst and calcite are also found. Occasionally, and rarely, a geode if found to contain the original water still trapped within. These rarities are called enhydros.

See cut & polished geodes
See amethyst 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)