A geode is a hollow rock lined with an inside layer of sparkling crystals. On the outside, geodesappear nondescript and are easily overlooked. However, cracking open a geode will display an arrayof colorful crystals. Searching for these hollow rocks can be an elusive pastime, that might be enjoyedby anyone who likes studying and collecting rocks and minerals.
The formation of geodes is complex, and the process occurs over an extensive period of time. Ageode begins as a hollow bubble located inside a layer of volcanic rock or another type of rock.These hollows could occur from tree roots or from animal burrows underground. With the formation ofthe rock, tiny air bubbles surrounded by the rock are initially hot. With the addition of moisture,chemicals are released into the water. Part of this mineral-rich water penetrates through the outersurface of the rock to the inside, which causes it to be trapped inside of the bubble. Tiny crystalsremain inside the bubble sticking to the sides. Over thousands and millions of years, layers forminside of the space. These layers include both agate and quartz. Some geodes have tightly packedcrystals inside of them, while others still have a cavity with crystals lining the interior surface.
Geode colors depend on the agate layer inside the rock and the type of crystals that form on theagate layer. Generally, the agate layer supplies the majority of the geode’s color. The color of theagate depends on how the minerals are distributed inside of the stone. If an abundance of iron oxideand cobalt are present, the colors will be red. If titanium is present, the colors will be in the bluefamily. Manganese produces pink coloration, while chromium and nickel result in green hues. Thequartz on top of the agate are generally either white or transparent, but quartz may have colorationas well.
Geodes may be found all over, but they tend to be most prevalent in desert regions. Anywherevolcanic ash is a part of the landscape and places containing large amounts of limestone are idealplaces to hunt for geodes. The State of Iowa has a significant geode deposit, even naming a statepark after the stone. A large cave in Ohio called the Crystal Cave offers tours for visitors wishing tolearn about geodes. Aside from desert and central regions in the United States, Mexico, Australia,and Brazil are common places where geodes are prevalent.
Vugs and Crystal Caves
Crystal caves and vugs are the locations where geodes tend to form. A vug is a cavity that occursinside rock with crystals lining the inside of the cavity. Vugs may form as a result of faulting or thecollapse of rock, and they often contain secondary minerals inside of them. Crystal caves are oftenhidden and blocked by many tons of rocks, making them difficult to find. Scientists theorize thatcrystal caves form when salty sea water evaporated millions of years ago, leaving behind salty layersof sediment that eventually hardened into a crystal.
Scientists and those who enjoy geology consider geodes to be hidden treasures, waiting to beuncovered. Virtually anyone can choose a nondescript rock to crack open and find out what is hidinginside. Every geode is unique, and they vary widely in color, and crystal formation. Geodes can alsobe a variety of sizes from small to quite large. Opening a geode involves tapping it with a hammer.Some people prefer using a rock saw to cut the rock in half. The rarest and most valuable geodescontain amethyst crystals and black calcite
Links to more Geode Information
Geodes: One of Nature’s Mysteries (PDF)
What are Geodes (PDF)
Formation of Geodes
How Geodes Form
Quartz and its Colored Varieties
Physical Properties of Minerals
Geode Crystal Formations
Geodes – A Very Cool Rock Formation
What Colors Are Geodes Naturally? (PDF)
Michigan’s Gem Stones (PDF)
Observing Rocks (PDF)
Where do Geodes Come From?
What are Geodes?
Celestine in Ohio (PDF)
What are Crystals?
10 Interesting Facts About Geodes
Geodes: Small Treasure Vaults in Illinois
Crystals: More Than Meets the Eye
Flint Hills-Rocks and Minerals
How to Hunt for Geodes
Geodes and Diamonds – Backyard Geology