Utah Grape Agate is one of the most unique rocks found in Utah. My vote for the most unique rock found in Utah is still Utah Agatized Barite. But Utah Grape Agate runs a close second. If it was just a pretty agate then it wouldn’t be such a unique rock. But Utah Grape Agate has a lot more going for it.
It’s the multiple personality rock! My essay, The Grapes of Sandra Fell, shows the two principle types side by side. And the replaced interior can be dramatically different. There’s an illustration of that too. (Full Disclosure: I wasn't there when my dad collected these rocks. Some might be from a completely different location.)
The red botryoidal “grapes” are the only red agate grapes known to occur in the world. The Indonesian grape agate grapes are purple. If you know of any that occur somewhere else, please let me know.
Utah Grape Agate is extremely rare. The location where they were found is well known and marked in virtually all Utah rockhounding guides. The down side of that is that the site has been picked clean for decades. A recent guide suggested that you take a screen with you and screen the sand for individual quarter-inch “grapes”.
In my blog linked above, I quote the only explanation for the formation of Utah Grape Agate that I have been able to find. Again, I appeal for anyone to help me out if there is a better explanation. No one has responded. But I don’t buy the explanation I found on the web and here’s why.
The “squashed grape” shown above is a solid chunk of rock. It had to have been squashed while the center was still flexible but the outside had hardened enough to crack. There is only one way – that I can think of – that could explain how that happened. It was ejected out of a volcano and landed while still liquid
Added to which, the one-and-only deposit of Utah “Plasma” jasper is relatively close to the San Rafael Swell grape agate site. (There is none of that left to find either.) Although Plasma is totally different in color and composition, it has one thing in common with Utah Grape Agate: It’s usually found in small, flat. oval shaped rocks.
Academic geology is a competitive job these days. If you choose to make your living that way, there are two options. (1) Work for a mining or oil company to find stuff they can extract to make money. (2) Specialize in a popular and interesting subject – for example “dinosaurs”.
I have yet to find a scientific geologist who knew much about the rocks that us rockhounds like. It just doesn’t pay to study agates and jaspers. I found a large and detailed explanation of the geology of "thundereggs" but it was written by an eccentric (but talented and dedicated) rockhound, not a credentialed geologist.
I found a meme a while back that tells it like it is:
“Money” drives a fancy red sports car. “Scientific investigation” takes the bus.
I’m not optimistic about finding a geologist who can explain Utah Grape Agate.
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