Friday's Rock - Geode Slice

kansuze (63)in #geology • 7 years ago (edited)

Many years ago on a trip to New York state, I was fascinated by all of the rock and mineral stores along our route. We stopped at a couple of them and that is where I first heard of geodes. In fact, for about $5 you could select from a pile of these round rocks and they would slice it open for you. To me, it seemed a little too much like "winning" a prize at the fair because maybe there was something good inside or maybe not.

So I passed up on having them slice one open and opted for this pretty specimen instead.

What is a geode?

a nodule of stone having a cavity lined with crystals or mineral matter

the cavity in a geode
~ Merriam-Webster

Geodes are usually formed within rounded cavities in sedimentary rock or left by gas bubbles in igneous rock. Over time, as the rock around the cavity hardens, the inside fills with dissolved silicates and/or carbonates from ground water or hydrothermal solutions. Impurities of different kinds color the crystals.

Softer surrounding rock can wear away completely, leaving these rock globes.

There are several basic forms. One type has large crystals and a large hollow in the center of the geode. Another type forms when microcrystals produce bands of color leaving a tiny hollow that has crystals around it. Still, a third type is completely solid with no hollow, and technically it is called a nodule.

Without seeing the rest of the slices, we can't tell for sure what type of geode formation it is - it's either the second or third type - but the rust colors are often an indication of iron oxides.