Utah Grape Agate is one of the unique red rocks found in Utah. They were ... You would be extremely fortunate to find anything now. ... found in the broken formations around the San Rafael reef. If you're not from around here, you would pronounce that, "San Raw-fay-el". But in southeast Utah, it's "Sandra Fell".
The individual nodules are usually small, but there's no rock I know of that looks quite like it. In fact, as nearly as I can tell, there are at least two different rocks ... roughly from the same area ... that are both called "grape agate". I'm not sure why ... maybe a Utah rockhound with better knowledge than me will leave a message and clear this up.
One geology web site has this about Utah grape agate:
"They weather out of sandy shale 'puddles' where they may have been a replacement of something in the sand. The puddles are always rounded or oval shaped and weather out of the shale at a slower rate than the shale around them, as if the agate is holding them together. The shale puddle always shows signs of being deformed as if the agate pushed down on the shale when it was settling. But the agate didn’t form until after the shale was deposited."
The name comes from the botryoidal red bumps that are found on the outer surface. You can see them clearly in the picture. They don’t look like grapes to me. I would have called one “measles agate”. The bumps often continue through the agate and as white dots. The one with larger red bumps might be "red currents agate". Here's a slice showing those white dots.
My theory about Utah grape agate is a little more detailed. I think this rock started as lava ejected from a volcano which landed as small plastic gobbets. I believe Utah plasma (see the blog essay here: Utah Plasma Jasper - Unknown Rock!) is the same thing but it landed in a different place and time so the resulting silica replacement is different. The shape and size of plasma and grape agate is almost identical. But the silica replacement result is totally different. If anyone knows more about Utah grape agate, please leave a message and let me know. The field is wide open for someone to do some great science because there are SO MANY rocks that just seem to be a mystery.
Like "barite", if you look up "grape agate" on the web, nearly all of the hits you will see is a completely different rock than the one we have in Utah. In the case of "grape agate" there is a very pretty purple rock that really does look a lot like a bunch of grapes. It's from a new mine in West Sulawesi, Indonesia. They stole our name. We had it first!
The very limited amount of Utah grape agate I found on the web is mostly low quality. My dad collected what I have over half a century ago when there was still quality material to be found. You are highly unlikely to find any like this out in the Sandra Fell today.
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