Zen Cat Pottery Plus

Sunday, April 22, 2018

How Geodes Are Formed

April 22, 2018

I recently had a special order for a heart necklace with an American Sign Language "I Love You" charm. I dutifully took out all my stone heart pendants, photographed them, and sent them to the customer. The one she chose is amazing! While working with it, I noticed its gorgeous variations and that got me thinking about how amazing geodes are! I know agate slice jewelry is super popular and trendy right now. Most of these slices are slices of geodes. So I thought I'd do a post about how geodes form.

First things first. This is a photo of the pendant she chose, and resulting necklace. Simple and beautiful.

You can find lots of beautiful natural stone heart necklaces in my shop, or convo me for a custom order!

Here are some other gorgeous agate hearts I have in stock right now:

I like holding them up to the light because agate is mostly translucent, and you can really see its gorgeous banding in the light.

But how do these amazing bands and shapes form?

Most form within a geode. Geodes can form in both igneous (volcanic) rock and sedimentary rock. They are most prevalent in igneous/volcanic rock, partially because there are a ton of ancient lava flows around the world. Here in America, old lava flows are the BEST places for geode-hunters to find them! So, I am going to discuss igneous rock geodes below.

I found this diagram here, and though it is not the best diagram, it is pretty good.

Basically, when a volcano erupts, gas bubbles get trapped in the lava. These gas bubbles form holes or cavities (called vesicles) in the cooled volcanic rock. As water (rain, streams, etc) flow over the lava rock, these cavities fill with fluids rich in dissolved and suspended quartz molecules and other minerals. Over time, these quartz molecules and minerals begin to form small crystals that attach to the inside walls of the cavity. These crystals are called druse.

As thousands (millions) of years pass, more water containing different amounts and types of quartz molecules and other minerals flow into the cavity and form layers. The original crystals (druse) get "filled in" and new crystals form on top of the new layers of agate. These layers of agate form bands that may "contrast" depending on their mineral content. This pattern of filling in layers, and forming crystals in the "center" space is repeated over hundreds, thousand, and even millions of years.

Due to its composition of quartz molecules (silica), the type of stone that forms in a geode is agate. Agate is a semi-precious gemstone. There are many different types of agate (dendritic agate, Brazilian agate, moss agate, crazy lace agate, etc.) besides geode agate. Most, but not all, agates are translucent. Most geode agates are translucent.

When you slice a geode open, you can find one that is entirely, partially, or slightly "filled in."

(above photos taken from here, here and here.)

This is a spectacular geode from Uruguay I found at Geologyin.com:

The largest AMETHYST geode ever found was also found in Uruguay:

It weighs 20,000 kg (That's 44,092.5 pounds or 22 tons, to us Americans). An average elephant only weighs 5,000 kg (~11,000 lbs) That's 5 elephants worth of stone!!!!

Here's a photo of it being transported and another with a man sitting in it!
(source: Verandah Magazine)

For jewelry, geode slices and carved shapes are often infused with color(s). This highlights their stunning banding and makes their crystal formations/shapes more visible. They are sliced, carved into shapes (hearts, wings, crosses, animals, etc.), or carved into beads.

I have some amazing geode agate necklaces available in my shop right now: