Geode Structure

There are three diferent kinds of "geodes." First there are the irregular shaped kind that formed in odd shaped openings under ground. Second are those that formed from silica at temperatures between 1,687 Degrees K and 3173 K. Third are the kind that formed from silica at temperatures above 3173 degrees K. That is the kind we are covering on these pages.

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Probably one of the best places in the world to study geodes is the Richardson Ranch near Madras Oregon. Their land includes the world famous Priday Plume Geode Beds, The Blue Beds, the Red Beds, and the Pie Beds.

This geode weathered naturally out of the "Blue Beds." That means that the obsidian, pitchstone, or rhyolite that it formed in has altered to soil. Notice the rope like section dividers. This is where the interior plate sections come together. The "Top Plate" is the largest, and always contains the geode central sphere. The side and bottom plates may have broken away in any order or quantity, but usually have some concave inner edge to show where it once connected to the inner sphere.

Cutting through the center of a geode parallel to the plane of the photo above, you can see the red spot that once was the very center, the surrounding radial spherical layers, and the sharp line where it pulled apart from the bottom plate. The outer points of the "star" connect to the exterior rope-like lines. The center filled with agate some time after the geode was formed.

These two geodes became attached shortly before cooling down to the transition point. The "Top Plate" on the left geode is on the bottom, and on top of the right geode, proving that gravity and top or bottom did not matter to them as they restructured to their cooler crystaline form. Also, note that the right geode contained more moisture than the left, and therefore filled with potch opal instead of agate as the left one did.

Some geodes have more weathered shell structures than others. I found two with the same outer features [even the same small plate just left of center] and removed the outer plates of the small one down to the quartz that filled and made a casting of the original inner hollow. Note the black line that passes through the cutting line on the second geode above. The line was also drawn on the small geode and dips down into the "dimple." The corresponding bump on the other side is often called the "pimple."

Click here to see a ''naked geode."