Making a Light-Up Geode From Concrete

Geodes are fascinating because they can look like ordinary rocks on the outside while hiding beautiful crystals inside. For this project, I wanted to recreate that idea using concrete, quartz, and amethyst. I made a rough concrete shell with a hollow interior, then filled it with crystals and added a light to make the inside glow. The result is a decorative geode that combines the rugged appearance of concrete with the natural beauty of quartz and amethyst.

Supplies

Concrete mix - 8 oz.

Water - 1 oz.

Decorative rocks / Crystals - 3 oz.

5 mm LED light - 1

AA Batteries - 2

AA Battery holder - 2

Hot glue - 10 sticks

3D-printed concrete mold

Tools

Mixing container

Mixing tool

Measuring cup

Pliers/wire cutters

Mold release

The first step was making the mold for the concrete geode. I wanted the outside to have an irregular, rock-like shape rather than looking like a perfect sphere, so I used a mold with an uneven surface. I also needed to leave enough room inside the mold for a hollow cavity where the quartz and amethyst would eventually be placed. To do this, I used the fuzzy skin mode for my slicing settings to make the surface irregular. I also made the mold non-spherical since nature isn't always perfect.

To create the cavity in the middle, I made a small negative mold.

To mix the concrete, I first found out the volume of my mold. I grabbed a measuring cup filled with water, then I filled the mold with the water and let the excess water run out. I then remeasured the water and prepared that amount of concrete.

To mix the concrete, first add a small amount of water to the mixing cup, then add the amount of concrete needed. Then slowly add more water in small amounts, and mix it together. The final consistency of the concrete should be like thick peanut butter.

First, spray the mold with some mold release, and make sure to get all of the surfaces so it will come off easier. Now the mold is ready to be filled; simply pour the concrete into the mold. After the concrete is added, put in the top mold and then tap the side of the mold to get rid of air bubbles.

After letting the concrete cure for a couple of days, lightly tap the mold with a hammer to release the concrete from it. Make sure to be careful with it; even though it is concrete, it can still break if you drop it.

After gathering your variety of rocks and gems, start adding the hot glue into the inside of the mold. You will want to use a good amount of glue to make sure the gems won't come out. Then start pressing the gems into the glue. Then fill the entire inside of this; make sure to have an even coat of gems throughout the entire inside.

To make it look better, I used a hot glue gun to melt and even out the glue coat, but this isn't necessary.

To make room for the LED, I grabbed a 1/8 inch carbide drill bit and drilled directly through the back of the geode. It took me a while to get through, but I tried to make the hole smooth. I then stuck the LED through the hole and hot-glued it into place.

After the LED was in place, I connected the LED leads to the battery circuit. When I made the battery circuit, I made sure the batteries were connected in series. This LED needs two AA batteries for enough volts to work. I tried to make the wires look orderly and hid the wires where I could.

To make the stand, I made a simple V stand in Fusion 360 and then made a negative copy of the geode and then subtracted it from the stand. I printed it out with some wood PLA and made sure the geode fit.

I am very happy with how this concrete geode turned out. What started as a simple concrete mold became a unique decorative piece with a rough, rock-like exterior and a colorful interior filled with quartz and amethyst. Adding the light helped highlight the crystals and made the geode even more interesting to look at. I also designed and 3D printed a custom stand in Fusion 360 to display the finished geode. This project was a fun way to experiment with concrete and natural materials, and I would definitely make another geode in the future with different crystals and colors.