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Geodes are small geological formations of incredible beauty. Geodes look like a usual stone from the outside but at the same time hide a real sparkling treasure inside. Geodes are not simple crystals; they are small geological models – a tiny cave in your hands. There are different types of crystals hidden inside.
The most common crystals found inside a geode are quartz and its purple variety amethyst. Less common are calcite, aragonite, celestine. Rare crystals of hematite, magnetite, pyrite, millerite, baryte, and rhodochrosite can be also found and are highly praised by professional mineral collectors.
Do you want to know how did the crystals get into the geode? Keep reading and you will be surprised what is similar between bubbles in a glass of champagne and geode formation.
If you are interested in checking out beautiful geode rocks you can find them by clicking here (Amazon link).
What Types of Crystals are Found in Geodes
Geodes are a generous source of different types of crystals. Minerals in geodes can be found in different appearances: in form of well-formed crystals with even facets which form beautiful scintillating druses, and in form of bands, layers, or crusts. The most common crystals you can find in geodes are quartz, amethyst, celestite, calcite, and selenite.
No doubt, the most common crystal found in geode is colorless quartz and amethyst (a vivid purple variety of quartz). They both tend to be geodes found in crystallized lava and connected to volcano activity. Quartz geodes usually display perfect scintillating druse of transparent perfectly formed crystals.
Most often amethyst geodes (they have a specific name because of not ideally round form – amygdule) come from Brazil and Uruguay (the Rio Grande do Sul region) and can be several meters in size!
Amethyst geodes are so popular that sometimes are sold in an unopened state in a Break-your-own geodes kit (Amazon link) together with a hammer. The person should feel him/herself as a mineral hunter and discover how beautiful a geode is with his own hands.
Another beautiful crystal found in geodes is Celestine. Its appealing light-blue color cannot leave you indifferent. Celestine is a strontium sulfate, so you will get not only a good-looking sample but also an intriguing chemical composition.
Well-formed calcite crystals can be also found in geodes. They look quite similar to transparent quartz, however, they have slightly different forms of facets and definitely, and calcite’s hardness is lower than quartz. Calcite looks nice in geodes and brings up feelings of an ice cave in your hands.
Gypsum crystals can be also found inside a geode. They can occur together with quartz; however, you will easily separate one from another. Selenite tends to form elongated needle-like crystals with a gentle sheen luster.
The biggest geode in the world – The Pulpí Geode in Spain is composed of giant (2 meters long) crystals of selenite). The rugby ball-shaped geode occupies a space of 10.7m2 (8m long, 1.8m wide, 1.7m average high). It is the only place in the world where you can get inside a giant geode.
TIP: The value of a geode mostly depends on the worth of what is inside of it. Check out the main factors of geodes value in the article below:
The Value of Geodes: Are They Worth Anything?
Rare Crystals Found in Geode
There are a lot of other crystals and minerals you can find in a geode. They can be not-so-appealing perfectly-formed sparkling crystals discussed above and can be observed in form of layers, crusts, and grape-like botryoidal crystals.
But it’s not the reason to skip this paragraph, because these rare crystals are highly praised by professional mineral collectors and can reach thousands of dollars for geode.
Rare crystals found in geodes are hematite, magnetite, pyrite, chalcopyrite, millerite, baryte, rhodochrosite, jasper, agate, and chalcedony, aragonite.
Agate, Chalcedony, and Jasper
These cryptocrystalline varieties of quartz are found as colorful layers inside the geode. They produce intriguing patterns and can tell a lot about the history of geode formation.
Calcite and its structural variety aragonite can be found both in the forms of crystal as described in the previous paragraph and in form of banding layers. Thin layers of aragonite can change each other inwards, creating gentle colorful stripes.
BTW: Do you want to know more about rock and mineral identification? The books listed below are the best ones you can find on the internet (Amazon links):
- Smithsonian Handbooks: Rocks & Minerals
- Gemstone & Crystal Properties (Quick Study Home)
- Ultimate Explorer Field Guide: Rocks and Minerals (National Geographic Kids)
Hematite and Magnetite
These two iron oxides are not as fancy as amethyst in geodes, however, they are highly praised by scientists and collectors as can shed a light on the formation of iron ore deposits and are commonly found during mineral exploration.
Pyrite and Chalcopyrite
Iron sulfide and iron-copper sulfide or another name “fool’s gold” look like real gold and can be an eye-catchy treasure in your collection.
This nickel sulfide occurs in form of thin acicular needle-like crystals resembling chrysanthemum flowers inside a geode. Millerite crystals are composed of nickel and sulfur.
The combination of such substantial elements produces fragile and thin crystals of a gentle-green color (rare for the crystal world). The specimen below is a goal of many crystal collectors.
Baryte and Rhodochrosite
These two crystals can be found together in a geode. The most interesting thing here is that a geode is usually a hollow left after an ancient shell, which is replaced by iron oxides and hydroxides. So everything is perfect here. You will have rare minerals inside an extremely rare geode.
TIP: If you have ever come across a mesmerizing geode with splendid color in a crystal shop, you may have wondered if that piece is dyed or natural. Find out the answer in the article below:
How to Tell if a Geode is Dyed: All You Need to Know
How do Geodes Form?
There are two types of geode formation processes.
The first one is gas bubbles or vesicle formation in basaltic lava during the uplift of magma from beneath the earth to the surface.
The physics of the process is the same as the immediate bubble formation in champagne after the bottle opening. The difference in pressure lets the gases in a dissolved state form vesicles.
Not all the bubbles can escape and when lava solidifies, bubbles stayed inside the rock providing a space for further crystal growth.
The second way is geode formation in sedimentary rocks. Geode can be formed in cavities left after fossil shells, branches, roots, and other organic materials decay leaving a void for the formation of crystalline materials inside a geode.
Eventually, the bedrock that holds the geodes weathers and breaks down. And if the geodes are made of highly resistant material such as quartz, chalcedony, or jasper they rest in irregular patterns at the surface.
TIP: Geodes and thundereggs may appear similar at the first glance. That’s why a lot of misunderstanding occurs in their differentiation. Find out more in the article below:
The 7 Main Differences Between Geode and ThunderEgg
How do Crystals Grow in Geodes?
Crystals form inside a geode because of passing hydrothermal solutions or groundwater precipitation. When hydrothermal fluids enriched in elements necessary for crystal growth or groundwater reach the voids in the rocks, mineral starts to crystallize.
Depending on the composition of the solution, different minerals will grow. If the solution brings silicates, the layers of chalcedony or jasper will inlay the geodes wall.
For example, silicon is essential for quartz and amethyst crystal formation, calcium and carbon are fundamental for calcite, while calcium together with sulfate is crucial for gypsum crystal growth.
Free space inside the geode lets the minerals inside the bubble form perfect crystals without any obstacles. Crystal growth occurs from the outer part of the geode towards the center.
Shifting in the chemical composition of the feeding solution will find its reflection in different colors of bands inside the geode.
In the end, the hollow interior of the geode is filled with inward-projecting crystals, new layers, or new generations of crystals growing on top of the old. And the whole growing process can last for millions of years.
TIP: You can find many geodes for sale on the Internet. But not all of them are real. Do you know how to distinguish the real from the fake? Read the article below and never make a mistake again:
What’s the Difference Between Real and Fake Geodes?
Geode is a perfect source of natural and perfectly-looking crystals. They can contain scintillating druses of quartz and amethyst, which are extremely popular all over the world, or they can hide a real mineral treasure for mineral collectors as milarite needle-like crystals of rhodochrosite and barite crystals inside an iron shell.
A lot of crystals can be found in a geode. We have discussed the most common 16 above.
They are quartz, amethyst, agate, chalcedony, jasper, calcite, aragonite, celestine, gypsum, hematite, magnetite, pyrite, chalcopyrite, millerite, baryte, rhodochrosite.
Geodes are essentially part of any mineral collection. They not only bear a druse of fine crystals but have an extremely interesting history of formation which definitely should be shared.
TIP: Geodes are mainly found near desert regions or areas with limestone deposits. Check out the best environments and locations in the USA in the article below:
Where to Find Geodes: 5 Best Environments & Locations (USA)