What is a Geode?
What is a Geode and How Does it Form?
A geode is a rock with a cavity that's lined with crystals. Crystals will be large enough to be seen with the naked eye.
The tips of the crystals in the cavity of the geode grow towards the void.
Although quartz is the most common type of crystal, crystals can form from several other minerals.
Geodes form mostly in limestone and in volcanic rocks.
In volcanic rocks the formation begins following an eruption. The lava that's spewed out of the vent of the volcano mixes with various gasses in the atmosphere. Some become trapped in the lava and form large bubbles.
When a volcanic eruption takes place beneath the ocean, water can become trapped in the lava. As the lava cools and solidifies the gas and water become locked within the newly formed stone.
The shape of the gas bubbles determines the shape of the geode. Over time as the water leaks out or dries up dissolved minerals which are left behind such as silica (silicon dioxide) begin to crystallise.
The reason some geodes have larger crystals is because they've had more time to grow.
When a significant amount of iron is present the crystals can change colour. For this to happen they must be exposed to heat which comes from radiation.
Most of the world's largest and most spectacular amethyst geodes are found in Uruguay.
Geodes can form within cracks, crevices and virtually any type of cavity in a rock. 
The type of crystals that form and their colour depends on the minerals being carried in the water.
Approximately one hundred and sixty million years ago in what today is Brazil, movement in tectonic plates and other geological occurrences created many deposits of quartz. The quartz was then exposed to radiation which turned the crystals purple.
The exterior of an unbroken geode will look like an ordinary rock. Once broken open however the cavity inside 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 size. Geodes can be a small stone or a huge boulder.
Some are hollow whilst others are partially or completely 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 in 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 probably the most popular type of geode. Although citrine is also well known the vast majority of geodes with citrine crystals are amethyst that has been heated.
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. All three are clickable and redirect to the original photos.
