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I just joined the group, and it looks like a great place for me to learn about rocks and minerals. I enjoy collecting them, but my knowledge of what I'm collecting is pretty minimal.

With that said, I have posted some photos of some pieces of what I'll call geodes in the Yellow Cat, Utah area. They are relatively small spheres when whole- about 1.5 - 2.0 inches in diameter.

I'm curious what these are and what the different layers are. The outside of most of them is a rough brown color. Then there's the red layer, sometimes banded. Then there's a clearish layer that either has a surface of small crystals or a bubbly surface. Then as you can see in the photos, there is sometimes a central "core" of a brownish material. If you expand the photos, you'll be able to see the texture of the central part.

I'm also curious how these form- if it's a series of periods with liquids filling a cavity with different minerals that precipitate out?

I'll get some photos of other rocks I picked up there, as I'll have questions on those too. Yellow Cat is definitely an interesting place with a huge variety of things to look for. I've still nt found any of the red wood limb casts.


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Hi Eric,

I don't know anything personally about the Yellow cat area but have a couple of thoughts:

1) If you are not already aware of it, mindat.org is a great place to try and determine what minerals occur at what localities. Here is their page for Yellow Cat, UT

2) The district appears to have a lot of neat (and some relatively rare) radioactive species reported, but I see nothing about quartz or agate, which is what your pictures look like to me. If they fluoresce, this may be indicative of something other than quartz/agate.

3) In general, you are essentially correct about your theory of how geodes form...

Thank you Tom. Thanks for the mindat.org link. Yes, it looks like there's some very interesting stuff there. I wonder if I should be much more carefull about what I pick up considering the radioactive materials in the area? It's surprising mindat.org doesn't list the quartz/agates. There is a ton of it there- in many different colors. And it was obvious from areas with a lot of chips that Native Americans used the area as a source for tools and points.

Will a black light be the right spectrum to check for flourescence?

Ideally, you want a combo lamp that selectively emits both short and long wave UV. I use this one.

There are many other reputable lamps from other manufacturers as well.

One of our members, Bill Jaeger, also sells lamps which I understand to be quite good and popular, although I have not used one of his myself. You might email him.

The only thing you don't want to do is buy one of those cheap flashlight led varieties commonly found on eBay. They are barely in the ultraviolet range and are almost completely useless.

THanks Tom. I'll look into those. Thanks also on the caution about the flashlight types.
Apparently, the Yellow Cat agate is petrified wood. Compare your finds to the pictures on this site:
https://www.westerncoloradopublishing.com/Utah_red_wood.html (I hope links are allowed here!)
I wouldn't totally discount your pieces as not being petrified wood yet, the structure of Yellow Cat wood doesn't seem to agree well with the more classic examples.

Radioactive minerals are an interesting niche in the collecting world, no worries as long as you follow a few simple rules such as washing your hands after handling, don't sleep next to specimens or carry them in your pocket, etc.
Good link Jack. It has great photos. I'll have to dig out some of my other pieces and see if I might have some of the red wood after all. And I'll try getting some photos of other material I brought back. Some of it is agate-like texture in grey/black/blue layers. There are some small outcroppings along a ridge with large sections of agate-like material. I'll get some photos of those pieces too and try to post those over the weekend.

I've added a couple more photos (Yellow Cat piece 1 & 2) of some different material. It has a very waxy texture and it fractures in a conchoidal fashion. The photo isn't very good, but the colors for piece 1 are primarily black/dark grey and a light blue- in layers. Piece 2 has some creme and almost pinkish colors mixed in with the blue. The spot it weathered out of seemed to be more of a layer or seam than a casting of wood, but I'm not certain about that.

If anyone is in to making cabs, I think this would make some pretty ones, and I'd be happy to send some pieces. And if anyone has thoughts on what this is, and how it formed, I'd be very interested.

Dear Eric;
From looking at your geode pieces, they ARE geodes; they have the agate rind & are hollow (no matter how little). You were exactly right about how they form. Unfortunately, all a UV will show is whether they are fluorescent or not, nothing else. MANY agates throughout the west are fluorescent, as well as the rare minerals (usually bright green). probably the best rule, for looking for rare minerals, out there, is size; if you can easily see an individual xl, it's likely not too rare! There are exceptions, as with any rule, but, it's still a good one to go by.
Your other pieces are jasperized something; possibly wood, but, likely a jasperized rhyolite (VERY flint-like).
Your friend, Steve
I have hunted Yellowcat District for over 10 years off and on. Yes, there are mini geode pieces out there, as well as agatized barite nodules. Have you found "Agate Valley" yet? It is west of Yellowcat, and a bit off the beaten path.... but just about every rock in that valley is a spectacular agate. I have seen every color of the rainbow there, as well as a few that were too big to pick up!

I have not seen any true Yellowcat redwood out there for a long time. You can see where ATV's have "grided" the desert out there. So the only chance would be to find a piece that washed up from the winter storms. There is some really cool radioactive, petrified wood out there... if you know where to look.
Thank you Dennis. I'm not familiar with Agate Valley- at least not by that name and what you described in the rocks. I'd be interested in knowing where it is and any suggestions about it. I'd also be interested in info on the agatized barite nodules. I haven't run across any of those.

Thanks for your input.
Drop me an email if you or others on this network are going to be in the Eastern Utah area. Agate Valley is one of those sites that has SO much material, that I don't mind showing it. I have many sites in this area, including Yellow Cat. I have found stuff that very few people have ever seen (IE: blue coprolites, red septarians, etc,). I even know of one site that has THOUSANDS of pounds of petrified wood just lying there. Last year I got a 80 pound agate nodule that has 5 different colors in it! Bet someone could make a few cabs out of that one, eh? Tis almost 3 foot in diameter. I will post photos of some of these items the next time I get over to my storage unit.
I'll definitely do that Dennis. I'm hoping to still get out there this Spring- before it gets too hot. Those locations you mentioned sound fantastic! And I'd love to see the photos when you get the chance.


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