Mad About Science: Geodes

By Brenden Bobby
Reader Columnist

Did you know that there are some rocks and gemstones that only exist on Earth? As far as we are aware, there is nowhere else for tens, hundreds, perhaps even thousands of lightyears away where these minerals exist in this configuration.

Before we get into today’s subject, it’s important to look at how rock is formed to get a better understanding of why and how gemstones and geodes exist.

Rock comes in three forms: igneous, sedimentary and metamorphic. Igneous rock begins its journey as molten rock in the Earth’s mantle that is pushed toward the surface through volcanism. Granite is a great example of igneous rock.

Sedimentary rock is formed when particles are deposited by air or water to form layers that fossilize into stone. A recognizable form of sedimentary rock is limestone, which is formed by the compressed skeletal remains of corals and other calcified critters.

Metamorphic rocks are formed by heat, pressure or chemical reactions that change the composition of the stone into something new. Limestone is sedimentary, but it can transform into marble when the limestone is subjected to immense pressure, forcing the calcium to recrystallize.

Many of the processes involved here don’t happen in the vacuum of space, except perhaps in the very early stages of a star system’s life. Limestone requires living organisms in order to form, but it’s not the only type of rock or gemstone that forms in the presence of life.

Geodes are very unique, not only in appearance but also the way they form. 

Courtesy photo.

The first step of a geode’s formation is air. A void must exist within an igneous rock in order to form a geode. This can happen when gaseous air is trapped inside of volcanic rock that is rapidly cooled on the outside, but cools more slowly on the inside. This causes ruptures and allows molten rock to leak out and leave a void of air in the center of the cooling stone. This can also happen in sedimentary rock where a more voluminous structure, such as a piece of coral, may decay and wither to create an empty space encased by solid rock. This process takes considerably longer than it does with igneous rocks.

It’s not immediately apparent when looking at a rock, but many rocks are porous at a microscopic level, similar to a tiny hard sponge. This means that minute amounts of water can travel through the rock and into the larger void within, and then out the other side. As water travels, it carries microscopic particles of other minerals with it, which end up getting trapped inside and begin to form crystal structures as more and more of these minerals are left inside.

Understandably, this process takes an extremely long time.

Due to the nature of a geode’s environment influencing its development, the types of crystals found within igneous geodes, versus sedimentary geodes, can be fairly predictable. Quartz crystals are more commonly found in igneous geodes, while silica crystals are often found within sedimentary varieties.

You may be wondering what causes some geodes to have color, while others appear milky or clear. Purple amethyst crystals are a telltale purple color due to the iron impurities deposited within the crystal. While it has a different name than milky white quartz, it’s the same mineral with a very similar structure, just colored differently by the impure iron. Red and pink amethyst also exist in these hues for the same reason, though irradiation can also play a role in the coloration of these crystals.

Agate found in geodes has a number of unique properties that make it distinct from the quartz crystals often found in these rocks. Groundwater laden with silica particles in acidic and PH-neutral areas will collect over thousands of years, depositing the silica in layers as it does so. It forms a microcrystal structure that appears very smooth to our eyes, as well as creating a unique banding pattern and often completely filling the entirety of the void found within the rock. In some cases, a geode will begin forming with larger crystals at the edges of the void, but high concentrations of silicates will begin to crystallize and fill the entirety of the void above these crystals to create a conversion into a solid chalcedony.  There are many types of agate crystals formed by this process with varying patterns of banding and coloration that act like geological time capsules.

These processes are only possible under proper conditions. The presence of liquid water is the most important and the rarest. As abundant as liquid water is on Earth, its presence in a liquid state is extremely rare in our solar system. Without a proper atmosphere to nourish a water cycle like the one we have on Earth, water molecules are prone to freezing into perpetual ice or being evaporated and whipped away into the vacuum of space by solar radiation. The presence of life is also necessary for certain geodes and crystals to form, whether it’s within fossils or voids left by tree roots or decaying coral — there is no place other than Earth where we’ve observed these things.

Keeping that in mind, there are gemstones that we know originate from space. Peridot is a green gemstone that’s actually made up of magnesium, iron and silica, and it’s been found in meteorites that have fallen to Earth. It’s also believed that the atmospheric pressure of both Jupiter and Saturn are so intense that they crush atmospheric carbon into graphite, then further press the graphite into diamonds, which rain down into the seas of liquid metallic hydrogen. Before you make plans to journey to Jupiter and dive for diamond sand, just keep in mind how much pressure must be involved to not only crush what is essentially pencil lead into a diamond, but to push hydrogen into a state of liquid metal.

Do diamonds float in hydrogen seas? I don’t know, but I think I just found my new band name.

Stay curious, 7B.