DIY Giant Concrete Geode

DIY Giant Concrete Geode

Remember collecting those shiny stones as a child?! They just seemed so precious. Well, now as a Ā grownup you canĀ challenge ‘Mother Nature’ by making your own withĀ this DIY Giant Concrete Geode tutorial. No wimpy stuff here, feel of real stone with this unique use of concrete. Check out my way of using easily found materialsĀ to get some super ‘bling’ in your life…

During all my ‘concreting’ I have discovered some great characteristics of this RapidSet Cementall that will aid in this tutorial.

Materials for the Casting:

  • RapidSet Cementall & Dust Mask
  • Reinforcing Fibres
  • Water
  • Container to mix in and mixing utensils (tongue depressors or like)
  • Smooth Plastic (plexiglass, CD case, or any flat shiny surface)
  • plastic bags/wrap/tape
  • cocktail straws
  • Vinyl tiles (xacto knife, straight edge/ruler)

Step 1: The Concrete Forms:

These geodes can be made in 2 versions. As a ‘Slice’ or as a ‘Hollow’ Geode.

To make the ‘Slice’ Geodes:

To make the slices you will need to ‘sandwich the concrete between some sheets of shiny plastic. I discovered that this concrete yields a super smooth shiny finish if it is poured one on a smooth surface. Be creative, even CD covers could work. I used some old floor protector mat pieces as plexiglass is expensive. Do not use glass as when youĀ put pressure on it you will break it (ask me how I know)

To give it a bit of extra reinforcement, I added a bit of fibres (but it could work without it). I have been impressed at the strength of this concrete, even as thin as 1/8″. RapidSetĀ also mixes slightly different as it absorbs the water quite quickly and I usually need to add more mix. It acts differently than Quikcrete. RapidSet has a plasticity to it and sets really quickly with only a few minutes (less than 10 minutes) of working time so be prepared.

Mix a slightly thicker; less runny version of this mix. You want to make a rough ‘ring’ of concrete on the plastic. Push it to keep the shape.

To facilitate the attachment later of a stand; add a straw into the top and bottom. Make sure it is hidden in the concrete.

Once you have the ring about the size you like (make it random shaped as nature is not perfect) place another sheet of plastic on top and wriggle it to flatten.

Do not flatten too much! It should be 3/16″ or a bit thicker. If it is very large; thicker is better. Mine are about 8″ or so. The edges SHOULD be quite rough, like stone.

All set to be great DIY Giant Concrete Geodes! Another great thing; this concrete sets in 1 hour!

To make the ‘Hollow’ Geodes:

Make a ‘lump’ out of some plastic bags or wet paper towels and wrap in plastic. Add tape to keep it secure.

Place the lump on the on the plastic and cover with concrete. Mound it and keep it rough to look like stone. You can sprinkle some sand if you like to add texture.

There you go, the ‘hard’ work is done. I love how they look like a slice of stone!

To make the square bases:

If you want to make a stand for your ‘slice’ you will need a base. My favourite material for shape making is the vinyl tile as it cuts easily and holds up to multiple uses. I made my blocks 3.5″ wide but you can use what size you like

A simple block shape is folded up and tape on 2 corners.

The other 2 corners will be opened so I made tabs to secure with clips (less taping)

Mix up some and insert a straw (end taped shut) into the middle and secure.

Awesome simple square bases (sponged with a bit of paint)

To Add the Finishing touches:

You will need:

The Layers:

If there are any fibres sticking out, you can singe them off with a lighter. Take some inspiration from some images of geodes. Keep it simple; it’s all about the layers. Some are ‘curvy’, some are ‘squiggly’, there are no rules. Nature is quiet random, so have fun with it. I liken it to doodling, quite relaxing…

Work light to dark or back and forth.

Add some metallics for fun; you are in charge here. The outer edge should look like stone so a thin layer of antiquing will do that.

The reason I prefer acrylic inks is that they are highly pigmented. InksĀ provide a lotĀ of colour without the ‘thickness’ that comes with cheap paint and they are permanent when dry so sealing isn’t a problem. Being a bit more expensiveĀ is worth itĀ in the long run.

Once you get the hang of the painting ‘layers’ you will be amazed! Add a few in gold or silver… Squiggle away…

The metallics catch the light so nicely!

Making the Crystals:

So, before you think I am crazy there is a reason for this! I had been thinking for so long about how to make ‘crystals’. Growing crystals is not easy and they don’t always last. Soooo, I did what I usually do, make them!

I threw a bunch of these flat glass beads in a cast iron pan (lid on just in case) and heated for a few minutes.

My idea is to shatter these. So slide them from the pan into some ice water and they will internally crack. Cool! Baked glass; who knew?!

I like the way that the glass breaks with a bit of a tap of a hammer. Just don’t do it on the table or on the new kitchen counter! No, I didn’t… btw. If you broke normal glass it would be too sharp and pointy.

So there you have the crystals that you will need to finish the DIY Giant Concrete Geodes and since they are glass, they will not dissolve…

Marbles will also break, and you will have larger and smaller pieces.

Attaching the Crystals:

The whole attraction to geodes is the crystal interiors! To adhere the crystals I tried a few different methods.

The most permanent way is to use epoxy (a 2-part glue that can cure in 5 minutes) but it will flow to places that you may not want. If you use epoxy you can put a piece of plastic that has been greased with vaseline on the back to allow removal once the epoxy has set.

Clear silicone will adhere the glass in a more rubbery way. It is supposed to be clear but is still a bit cloudy once dry. It is quoted as being waterproof however.

The quickest and easiest method is with hot glue. It allows you to set the stones where you like and build up quickly. Ask your self how permanent or much wear it will get. I intend to use them as decor so that will be adequate for me.

Build up the ‘crystals’ and add stones to make it look authentic.

Blue glass with a bit of clear mixed in. Layer and fill but keep within the shape. The colour options are endless…

That is one huge geode!

The back side gets a bit of antiquing.

To give them some shine you can add a layer of gloss medium. The metallics will catch the light so I like the shimmer of matt against shine. The concrete has quite a shiny finish on it’s own.

For my ‘blue’ room with a stand made of copper wire.

Insert a piece of wire (coat hanger) into the slice and base, adhere with glue.Ā Baffle your friends!

So many options.

They go great with the rustic concrete bowls! Don’t blame me if you catch yourself staring at them for too long… Bravo!

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  1. Hi!
    I LOVE these!! I am going to try these today and was wondering at what temperature do you “fry” your gem stones to make them crack? Thanks for the inspiration!!

  2. I love these so much!! Beautiful!
    I was just wondering about how you let the cement set. Do you leave the cement between the two pieces of plastic/plexiglass while itā€™s setting for an hour, or will it stick to the plastic?

  3. All the steps were clear and looked easy to make. I have another suggestion:How do you think if we make the main frame of resin?(instead of concrete). Drawing two irregular rings inside each other with hot glue,then fill it with resin and then add some crystal stones to the inner ring after resin was dried.

    1. Yes, I have seen many do that. I think they use silicone caulking as the edge. I was just marvelling at how it feels like stone when done and shiny surface as well. Happy making!

    1. The straw is just a way to keep a hole to allow some wire to be inserted later. If you want to use a regular plastic one, or larger it will work. Then you have a way of being able to make a stand for it. Hope that helps!

    2. A regular straw will be fine. It’s just a way to keep a hole to insert the wire to make a stand later. Even a larger one or other tube would work! Hope that helps!

  4. Hi Barb!
    I love your creations. I just thought I would share a tip with you: 2 part epoxy will not stick to the sticky side of tape. So, in stead of going with messy Vaseline coated stuff, you can just slap some painters tape or packing tape on there! Keep up the fabulous work!

  5. In the event that you can’t get to cement you can always your that air dry clay! Get a big chunk and hollow it out and basically do all that you already did.
    Thank you for these tips!

  6. hi barb! hoping you see this fairly soon. iā€™ve never worked with concrete, in any way shape or form. my kindergartener is obsessed with geodes and wanted to do this as his art fair project. we did not have the type of concrete you used around here. we used quickcrete. i noticed as it hardens, thereā€™s a ton of moisture coming out. how long does it take to fully dry? i have it in a small bathroom with a heater going. iā€™m wondering if this will affect the painting and glueing portion of the project. any other tips would be great! thank you so much for this idea!

    1. Oh, dear! Quickrete is a brand, not sure which you used. Concrete needs to ‘cure’ not really dry. The reaction happens and then it will seem to dry. Some of the regular setting mixes do take a couple days to cure. If it feels wet it is still not ready to be covered with paint. Have you poked it to see if it is actually very hard yet? This post may help…