I bought copper tubing from the local hardware store which I bent into an oval frame, and I used an embroidery hoop for the center.
Then came the fun part! I experimented with different yarn thicknesses to mimic texture inside a geode. It doesn't quite look like a geode in the end (we call it our "big red eyeball"), but I think it adds the perfect bit of kookiness to our space.
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Wedge