Geodes

All About Geodes

What Are Geodes?

    Geodes are often hollow, spherical minerals filled with crystals. Geodes are referred to as "Variable Phenomenon",which means that there are many different ways they can be formed. "Geode" derives from the Greek word "Geoides", meaning "earthlike". Geodes are hollow, round rocks lined with crystals. Sometimes, nodules can be often mistaken with geodes. Despite them being nearly the same, the difference between the two is that nodules are completely filled with crystals, whilst geodes are hollow and lined with crystals.

A Nodule

How Are Geodes Formed?

    Geodes most often start as bubbles in volcanic rock, but can also be created from tree roots or animal burrows in sedimentary rock. Other times, the cavity could form because of a fossil being dissolved, or from decaying organic matter that leaves a gaseous pocket. 

    Over time, the shell of the geode hardens, and water that contains silica precipitate forms on the walls of the hollow geode. Layers of the silica water cool and form crystals inside the geode. Different silicas cool at different temperatures, creating layers of crystals. The most common crystals found in geodes are quartz, but amethyst and calcite geodes can be also be found. 

My large amethyst geode with 
a chunk of calcite inside, along with
other geodes and a nodule in my collection.

    Every geode has its own uniqueness in composition, and it can only be fully seen when cracked open or cut. The different sizes, compositions, and shades of color make each geode special. The shell of a geode gives no indication as to what crystals and formations are hidden inside, so opening a geode is always an exhilarating and fun thing to experience. The most prized geodes are usually those with black calcite or amethyst crystals.

Where Can You Find Geodes?

    Most geodes are found in large numbers in certain areas. These areas are usually near volcanic activity (or at least what used to be volcanic activity), since lava flows can be huge and are honeycombed with cavities and "lava tubes" left by gas bubbles. If the conditions are right, which in volcanic areas they usually are, large geode beds can be formed. One area like this is in Dugway Geode Beds in Utah. In fact, the Great Salt Lake is a remnant of an ancient lake called Bonneville. At one point, Bonneville covered most of Utah, and the waves produced in the lake helped break loose the geodes in the geode bed. 

    Brazil is also known for its geodes, specifically their amethyst geodes. In southern Brazil, there's an area called Rio Grande do Sul which are massive flood basalt lava beds. They were formed when South America split from Africa, about 150-160 million years ago. Many amazing amethyst geodes are found in Rio Grande do Sul, including ones taller than a fully grown man! 

    You can always take a trip out to Utah or Brazil, or you could go online or a local mineral store and purchase some! These are fun to open, usually only requiring a sock, hammer, and some sort of chisel. You place the geode into the sock, then you place the chisel on about where the middle is, and tap it with the hammer until you feel it split. Once you feel it open, take a look and you should see the crystal structure. You can go one step further and sand the area where it split, but that's a personal preference as some people like the raw split look. 

A Split Geode