A recipe for Geodes

Geode split in twoWhere do geodes come from?

Geodes are like magical secret caves, ordinary, rock-like spheres on the outside but sparkly and colourful on the inside; all hidden and mysterious! But how are these wonders of nature created? Let’s have a look at the earths recipe:


It all starts with empty spaces or cavities in rocks. These are normally created either by air bubbles in cooling lava or when lava flows into water and the outside cools before the inside, forming a hard shell which then cracks and lets water inside. Geodes can also form in sedimentary rocks such as limestone or sandstone. Wood or small animals which have been buried rot away over thousands of years leaving a small cavity is another option.

Add some water

The next stage is for mineral-rich water to seep through the porous rock and fill the cavity. This water leaves a thin layer of minerals clinging to the inside of the cavity (a bit like when you’ve been swimming in the sea and later that day you can still taste the salt crystals on your skin) and over thousands of years these layers of minerals build up forming crystals that eventually fill the cavity.

Spice it up a little

If we throw in a trace of iron, magnesium or copper into the water which is swooshing around inside the bubble in our rock then voila, you have color variation. The presence of these minerals can alter the colour of the crystals in our geode significantly, for example, amethyst becomes purple from iron.


If our geode was close to a source of heat while it was growing that would change the color, too. Amethyst which is heated a lot loses its purple color and becomes a soft yellow (which is citrine).

Leave to Stand for a few million years

How long it takes to form depends on the size of the geode; some are small enough to fit in the palm of your hand or to catch the light while perched on your windowsill. At the other end of the scale, the largest gypsum crystals, from a cave in Naica, Mexico, have grown up to 36 feet long – yep, that cave is actually just a very large geode!!


How to crack open a Geode

If you’ve discovered or bought a whole, unopened geode you’ll want to crack it open as safely and neatly as possible. All geodes are completely unique, and could contain anything from clear, pure quartz crystals to deep purple amethyst crystals, or perhaps agate, chalcedony or citrine. Fortunately, there’s more than one way to crack open a geode…

N.B. Put on safety glasses before attempting to open a geode.

1. Hammer
Take a small hammer (preferably a proper rock hammer not your average household claw hammer) and strike the top center of the geode. It will probably break into a few pieces but it’s the best way for kids, though not perhaps recommended for particularly valuable or rare geodes.

2. Chisel
If you have a masonry chisel you can work your way around the geode tapping gently in a neat line, just scoring the rock.  Keep going round until the rock splits open. Patience is definitely the key here!

3. Soil pipe cutter
This common plumber’s tool is a bit like a bicycle chain on the end of a pair of pliers and can be brilliant for splitting a geode symmetrically into two equal halves. You simply wrap the chain around the geode then squeeze the handle. Hopefully it should crack smoothly all around.

4. Diamond Saw
If you’re really rich you can use a lapidary diamond-blade cutting saw to slice the geode in half. (Note that oil can damage the interior of some geodes).

Article by Jem on 23 November 2014

Recent Articles