What is a Geode?

What is a Geode and How Does it Form?

A geode is a geological structure with a cavity lined with crystals that are large enough to be seen with the naked eye.

The tips of the crystals grow inward from the walls of the cavity, pointing toward the hollow centre.

Although quartz is the most common type of crystal, crystals can also form from several other minerals.

Geodes form mostly in limestone and volcanic rocks.

In volcanic rock, the process often begins with lava. Lava contains dissolved gases, which can become trapped as bubbles during cooling. As the lava solidifies, these bubbles leave hollow cavities in the rock. These cavities, correctly known as vugs, can later fill with mineral-rich fluids, which, over time, may become geodes.

When a volcanic eruption occurs beneath the ocean, seawater can also interact with the lava, and steam may become trapped inside. As the lava cools and solidifies, the trapped gas and steam leave cavities in the newly formed rock.
The shape of the cavity determines the eventual shape of the geode. Over time, mineral-rich water seeps into the hollow space. As the water evaporates or drains away, dissolved minerals, such as silica (silicon dioxide), begin to crystallise on the inner walls.

Early crystals are usually clear or milky quartz because they're composed of pure silica. As growth continues, the environment changes. When iron becomes present in higher concentrations, and the cavity is exposed to heat from natural radiation from surrounding rocks, the quartz crystals turn purple.

Some geodes have larger crystals because they've had more time to grow.

Most of the world's largest and most spectacular amethyst geodes are from Brazil and Uruguay.

Geodes can form in gas bubbles, cracks, or virtually any cavity within a rock where mineral-rich water can collect and crystallise over time.
The type and colour of crystals that form in a geode depend on the minerals dissolved in the water that enters the cavity.

Around 160 million years ago, volcanic activity created massive lava flows in what is now Brazil. As the lava cooled, gas bubbles became trapped inside. These bubbles became hollow pockets that later filled with mineral-rich fluids.

The exterior of an unbroken geode often looks like an ordinary rock, but once opened, the cavity can be quite spectacular.
Geodes can be found all over the world but some of the most notable locations include the United States (specifically the Midwest), Mexico, Brazil and South Africa.

They can be any shape and range in size from a small stone to a huge boulder.

Some geodes are hollow, while others are partially or entirely filled with crystals.

The town of Keokuk in the U.S state of Iowa has an abundance of geodes. The deposit dates back approximately three hundred and fifty million years.

Geodes from the Las Choyas deposit in Chihuahua, Mexico, are mined as deep as one hundred feet underground. Hundreds of kilos of geodes are mined in this remote stretch of desert.

Amethyst is the most popular geode. Citrine is also well known but most geodes with citrine crystals are heated amethyst.

Article Photos

The geodes in the first and third photo in our article were photographed by Stan Celestian.

The second photo was taken by James St. John.  

The amethyst geode is from our collection.

All photos are clickable and redirect to the original image.