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Geodes - rocks with secrets

Geodes - rocks with secrets

If you’re anything like me you will have noticed the recent surge in geode themed objects.

These gorgeous balls of crystals have inspired everything from cakes to lip and nail art, with real examples of the material showing up everywhere from decor to the kitchen sink (I desperately need this in my life even though cleaning it worries me!).

But what are these geodes, and why are people so inspired by them?

A geode is a hollow rounded form filled with sparkling crystals. It would have started life as an empty cavity in a rock formation, which grew layers of crystals over hundreds of thousands of years.

How does rock get a hole in it?

These cavities aren’t anything to do with dentists, although there may have been some initial rot to begin with. Usually the starting point for a geode is a bubble. This would have formed in association with a volcano, when molten lava was spilling out from it.  

As the lava cooled, bubbles would have been trapped as the molten rock solidified around them. Think about the bubbles you see in ice or glass. It’s exactly the same thing, only with rock.


In other situations, tree roots or the burrows of prehistoric animals might have been buried under layers of sediment, which, with the passing of time and the pressure of the layers above, solidified into rock. In this case the tree roots would slowly rot away leaving a hollow or cavity behind.

So what happens next?

Over the following centuries, there would have been a few rainy days. Water trickled down through the layers of rock above these cavities, dissolving and collecting minerals as it travelled. Once it got to this gap in the rocks it would have filled the empty space for a while, leaving those minerals behind as a souvenir of its presence.


Wait –how does water dissolve minerals? Is my marble worktop safe??

As you’ll know from experimenting in the kitchen, hot water is able to dissolve and hold much more material than cold water. Once the hot water cools down, some of the materials held within it are forced to crystallize back into their solid forms because the water can’t hold them in solution any longer.

You might also remember situations where you might have created something called a super-saturated solution. This is when you put so much sugar in your tea that there are still solid crystals of sugar at the bottom. There is only so much material that a quantity of water can contain.  Once it reaches that point, it physically cannot dissolve any more. This is called a solubility limit.  

The hotter the water is, the more material it can dissolve – both in quantity and in types of material.

If you leave water on your marble worktop it won’t dissolve. In normal circumstances, water on its own can’t dissolve minerals like this. You'd need to add some form of acid. Even if you pour boiling water on the surface it won’t affect it. So how does a rock dissolve in water?

In atmospheric pressure (what we live in on the surface of the Earth), the boiling point of water is 100*C. After this point, the water turns into vapour (steam). If we increase the pressure the water is under, that vapourisation point increases, meaning that we can make water even hotter before it becomes a gas.

Once you heat water up under pressure, it is able to dissolve pretty much anything. Including rocks. We call this a hydrothermal (hot water) process. Water truly is the greatest solvent on Earth.

Okay, that’s enough about water. Now tell me how the crystals grow!

The majority of the rocks on the surface of the earth are made up of silica. Silica (a compound of silicon and oxygen) accounts for 59% of the surface of our earth, and makes up more than 95% of all our known rocks and minerals. This means that there’s lots of it around for the water to dissolve on its journey down to that empty bubble in the lava.

The super-heated water carrying these materials slowly fills up this empty chamber in the rock formation. At this point a couple of things can happen. Either the pressure or the temperature might change, meaning that clusters of silicon and oxygen atoms have to crystallize out of the water solution as silica.

Pure silica is better known as rock crystal quartz – a colourless, transparent gemstone that is relatively abundant around the world.  


So if it’s just rock crystal in there, why are some of them coloured?

As water travels down through the surface of the earth, it picks up a bit more than just silicon and oxygen. There are a number of unusual elements that are rarer on the surface of the earth than down below. When combined with silica they create colours in a gemstone. These are known as colouring elements – or if you want to be posh – transition elements.  

Elements like iron, manganese, chromium, copper and nickel can all contribute different colours to gemstones. These crystallize out of the hydrothermal solution with the silica to give colour to our gemstones. Generally speaking though, silica prefers to get on with things without involving these elements. They're usually the last ones picked for the team and are often used as replacement atoms within the structure when it’s finishing off the job. That’s why the prettiest colours are usually at the top of a crystal.


Okay – but what’s going on underneath these crystals? There are bands of material that don’t look anything like crystals!

These coloured bands are what is known as polycrystalline quartz. We call this material chalcedony. In chalcedony, there are loads of tiny crystals which have all grown in different directions. They’re packed in so tightly that we can’t see where one ends and the other begins. The differences in colour and transparency are because the water contains different amounts of transition elements with each layer. It’s like counting tree rings. Each change in layer indicates a new solution of materials.


So how do these lovely things make it to the surface?

One of the things we can rely on in this world is the changeability of the weather. As seasons come and go, water freezes in tiny cracks in rocks and as it turns into ice it expands. This widens the crack and weakens the surrounding material. Eventually the rock breaks apart. Over years of erosion, eventually geodes break through to the surface of our planet. 

Why doesn’t the geode erode too?

Quartz is a relatively hard material, and that polycrystalline structure is very tough and will resist breakage because of all those microscopic interlocking crystals. As a result, the geode will survive much longer than the volcanic or sedimentary rocks surrounding it. These rocks don’t possess the same durability properties, so are much more vulnerable to the actions of wind and rain.

So then someone just picks them up and hits them with a hammer?

Yes. Pretty much.

Sometimes a little bit more care is taken and they are cut into slices and polished into slabs, but there’s no way of telling what’s inside a geode until it’s been opened. It’s like a mystery package from the world itself.


So are all these colours natural?

Not always. Sometimes a geode is opened up and it’s just grey inside. The banding might be really pretty but the colours are muted and a bit dull. It’s quite typical in South American agates (concentrically banded chalcedony) and geodes. That’s when scientists get a bit clever with dyes.

A combination of acid and metal solutions are used, and the stone is frequently heated to fix the dye in place and alter the colour seen. The more strongly white layers of the banded agate or geode don’t absorb dye very well – they tend to be less porous than the grey areas.  


Can we tell the difference between dyed and undyed material by eye?

It can be really hard to spot whether a stone has been dyed or not without resorting to laboratory equipment. Because the structure of chalcedony is so very fine, we can’t always see concentration of colour in cracks in the material – even with powerful magnification.

But there are some popular dyes out there which are so blatantly unnatural that they’re easy to spot. Anything neon could not possibly occur in a natural quartz geode, so we can reasonably say that the material has been dyed.

So why are people so inspired by these geodes?

Well. It’s a surprise parcel from the planet whose matt surface hides an abundance of colourful sparkling crystals inside. What’s not to be inspired about? Perhaps the fact that they have a dull exterior that disguises their colourful and sparkling soul is something that people can identify with and therefore appreciate more.  

At the end of the day, once broken or polished into slices, they’re just gosh darn pretty.

With geodes as with people, there’s always more beneath the surface.


To see more geode inspired goodies, have a look at our Pinterest board - Jewel Inspired!

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