This beautiful specimen is called a Thunderegg from Oregon. I have quite a few nice one's, but this is my personal favorite. The orbs in the agatized center makes it very unique.
On the outside it looks like an ordinary rock. When you cut into it, you will see how beautiful it is. They mostly have a solid agate center with different color combinations possible. 
The best part about cutting into them is not knowing what it will look like inside. I love the suprise factor involved. Some aren't quite as exiting inside, but they are all beautiful.
The best places to find these are on private land. If you search Thundereggs on the net, there is a couple of ranches that will let you dig, for a small fee. 
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So how do I recognize those rocks? Can we find them anywhere or just in places with suitable geological history?
Oregon in the United Stated is where they are from.
Are they like geodes? They are definitely unique.
Very similar to geodes. It's just rare that a Thunderegg forms with geode crystals inside.
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Never heard of those. They are beautiful though.
Thank you. They are really fun to cut into.
Nice post
Love thundereggs.... guess it's a requirement being our state rock here in Oregon. Planning on a future post to highlight how fun digging for them can be. It's quite addicting. Where did you find the one in the first pic...it's a beauty! Madras by any chance?
I bought it at a thrift store in California about twenty years ago. I believe it was mined in the 60's, cause the bottom is covered with this green felt material used around then... I've only found one hunting for rocks in California. Odd enough to find one here in a creek. It was even odder that it had already been cut. I assume somebody lost it in the floods. Hopefully it just washed there yard stash of rocks away and not there whole house. Kinda sad after I thought about how it may have got there.
You could always pretend that it came from someone like me who just goes and randomly drops the overabundance of their rock hoard back into nature, lol.
You should include Richardson Ranch in Madras on your future Oregon rockhounding adventure. It's the thunderegg capital of Oregon, lol, and they will cut your finds right there for a very nominal fee. So worth the trip and isn't too far from Glass Butte where the obsidian is.
I've always wanted to check it out. The only pay to dig place I've ever been to was actually a gold mine. That was a pretty fun trip too. I want to hit the coast up there I think near Bandon was where these great agates came from my moms friend gave me. Real clear with good banding but not many colors. Beautiful stuff though.
Yeah...definitely include beachcombing in your itinerary! This is my fave agate that I've found most recently.... It was wedged between two rocks but my other half somehow got it free. Unfortunately the pic doesn't do it justice.
