Geodes Or Rock Candies | Formation of a Geodes | ScienceMonk

“Geodes are the secondary geological formation within sedimentary and volcanic rocks. They are nearly spherical rocks having hollow cavities lined with crystals.”


The crystals are formed by the precipitation of minerals by hydrothermal fluids, groundwater or genetic water resulting in an inward concentric growth of minerals inside the hollow cavity.

Geodes are nature’s magical way of surprising us by saying hey, crack open my gift to see the most colourful and beautiful treasure I have for you. They pop up as a surprise to people who are not familiar with their formation.

Many people collect geodes for their sheer beauty and attractiveness. An unopened geode looks just like any other rock, however, on being cracked open, they show us a whole new world of beautiful crystals and agate bands.

These colourful rock crystals form by the continuous work of nature that may span for a few hundreds of years to a mighty million years. So let’s find out how these marvellous rocks form.

Formation of a Geodes

Geodes are usually found in geochemical environments influencing the chemical precipitation of minerals.

They are generally located in

  • Stratified volcanic deposits of basalt and tuff, or
  • Stratified sedimentary carbonate deposits like limestones and dolomites.

Geodes in stratified volcanic rocks form in particular geochemical environments where hollow bubbles from inside the layers of rocks. These bubbles can develop from the air inside explosive volcanic rocks or by the burrowing activity of animals and other biogenic activities.

As rainwater trickles through these hot bubbles, the chemicals present in the volcanic rocks react with water to form chemically active fluids. These fluids are soaked by the hard outer layer of the bubble and even penetrates inside it. The mineral-rich fluid keeps passing through the bubble and slowly and gradually builds tiny crystals or micro-crystalline bands on the inside of the bubble.

Formation of a Geodes

The continuous activity of water over hundreds to millions of years precipitates more minerals which fill the hollow centre through concentric inward growth. This results in more prominent layers of beautifully coloured crystals that look no different than a candy centre. Most geodes have interiors made of Calcite (CaCO3) or Quartz (SiO2).

Agate geodes are the most common geodes found on earth as they are a variety of quartz (SiO2) which are silicates. Also, since 90% of the rocks found on the earth’s crust are silicates or have silicates in their structure thus geodes having as a centre are more common.

Geodes formed in sedimentary rocks like limestones, dolomites, and calcareous shale are usually smaller than their volcanic counterparts. Gas-filled voids in sedimentary rocks or shells, tree roots, branches and the burrowing activity of animals often leave a cavity for the chemical precipitation of minerals. These cavities get filled with quartz, opal, agate or carbonate minerals.

The Secret Behind Their Vibrant Colors

Geodes have various colours like blue, pink, purple and red that form due to the different minerals present in the chemically active water. Even traces of elements like iron, sulfur, copper or magnesium in water can give different colours.

The deep purple colour in amethyst is a result of iron content in water, the pink colour of the dolomitic layer is due to the presence of calcite, magnesium and a minor amount of manganese.

Formation of a Geode

What Does A Geode Worth?

These pieces of nature’s artwork are collectable and are priced at 30$ to 500$. Geodes are sold at various auctions for art galleries collectors museums and even at various high-end gift shops. Their price depends on their colour saturation, tone, the symmetry of the outside shape of the geode and size.

The crystals with deep tones and intense colour saturation are priced high. The superiority of an amethyst decreases if the amethyst crystals lose their colour due to weathering and become dull or have reddish-purple hues. Amethyst can even lose its purple colour and become soft yellow or citrine when exposed to very high temperature.


The most beautiful quality geodes are found in Brazil, Siberia, USA, and Mexico. Geodes have unstable host rocks like calcite, tuff, and basalt that are vulnerable to weathering so, they can be easily collected when their host rocks have weathered away. Some geodes are produced by mining the host rock, but that method is difficult, costly, and often damages the geode.

Read More- Paleomagnetism

Every little geode has a tale to tell. The beautiful coloured crystals that you see might have taken millions of years to form. So adore this art piece of nature for its sheer beauty and nature’s perseverance at its best.

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