Can someone identify these rocks?

November 17, 2009 7:37 PM

Can anyone identify these two rocks from near Hatch, Utah? Picture links included inside. One we're just wondering about. For the other, we're stumped, and the geologist we've asked had never seen a rock like it.

We're just curious about this one.

http://www.hardsun.net/stone_pictures/img_8046.jpg
http://www.hardsun.net/stone_pictures/img_8047.jpg

This is the puzzling one. It came from the same area. The owners of the lot took it to a Bureau of Land Management geologist, who said he hadn't seen a stone where the ball and the surrounding materiel are the same. The ball pops out, and there's more nodules inside the stone with more stone balls.

http://www.hardsun.net/stone_pictures/img_8043.jpg
http://www.hardsun.net/stone_pictures/img_8048.jpg

Here's another stone of the same type, but more weathered. You can see an intact nodule, and another nodule that is partly open, showing the ball inside.

http://www.hardsun.net/stone_pictures/img_8044.jpg
http://www.hardsun.net/stone_pictures/img_8045.jpg

posted by dragoon to Science & Nature (16 answers total)

Ignore the woo in this link - this is the important bit: Moqui Marbles (also known as Shaman Stones or Thunderballs) are sedimentary concretions. They form as sediments are laid down at the bottom of bodies of water. The moqui marbles have harder minerals than the normal sediment. The sediment layers then turn to sandstone. When the sandstone erodes away, the marbles are uncovered.

I found that link when I came across a reference to Utah in this piece about Mars "blueberries."

IANAGeologist. I just like rocks. Those are pretty cool, whatever they are.
posted by rtha at 7:51 PM on November 17, 2009

I'm thinking it's a concretion in some kind of sandstone. I agree, cool rock whatever it is.
posted by pappy at 7:57 PM on November 17, 2009

The first one looks like flint to me.

The second one is REALLY COOL.
posted by elsietheeel at 8:01 PM on November 17, 2009

BTW, it looks like geologic maps can be found here, which might help you identify the rock type if you can pinipoint where it was found.
posted by pappy at 8:03 PM on November 17, 2009

Best answer: I agree that the second one is some really awesome perfectly round concretions in a sandstone. Concretions are the same material as the rest of the rock except with pore spaces between the grains filled with a another mineral known as the cement so its not odd that they look like the same material as the rest of the rock.

The first one looks very much like some kind of chert to me (which flint is) but it could be some kind of crazy carbonate. see if you can scratch that polished surace with a nail and if you cant I am pretty happy calling it chert.

Nice rocks. I am super jealous of the concretions.
posted by DanielDManiel at 8:10 PM on November 17, 2009 [1 favorite]

Oh, and I have a masters in geology, but did my research in paleo so I may not be right. Someone else could probably tell you way more.
posted by DanielDManiel at 8:13 PM on November 17, 2009

Response by poster: After looking further, we think the first is variscite - it can scratch glass, so we're thinking it's a gem, and it looks fairly similar to the images we can find online.

Moqui marbles are full of iron and they're fairly hard. The strange thing about these balls is they're soft sandstone, just like the surrounding rock.

Another one was broken a while ago, and it appeared to be just pure sandstone inside.

Thanks to everyone for identifying the second one as a concretion.

pappy, thanks for the geologic maps - we'll try to find the lot on it.
posted by dragoon at 8:22 PM on November 17, 2009

After looking further, we think the first is variscite - it can scratch glass, so we're thinking it's a gem, and it looks fairly similar to the images we can find online.

I had to look it up, but if it scratches glass it is not variscite which has a hardness of 4-5. Chert however is right around or slightly harder than glass so I am sticking to it.
posted by DanielDManiel at 8:33 PM on November 17, 2009

Oops, glass is ~5.5 and chert is 7 (I thought it was slightly less than macrocrystaline quartz but was wrong) on the Mohs hardness scale so chert is definitely harder than glass and variscite is still definitely not. Still going with chert.
posted by DanielDManiel at 8:53 PM on November 17, 2009

Seconding Daniel on the chert reasoning. In my father's rockshop I saw lots of concretions but never any that were so perfectly spherical. Some concretions were the same material as the matrix and some appeared to be of a different material.
posted by X4ster at 9:21 PM on November 17, 2009

Response by poster: So I think we've been convinced it's chert - the nail did not scratch it.

Would this be jasper? The wikipedia picture of green jasper does resemble what's sitting here.
posted by dragoon at 9:33 PM on November 17, 2009

nthing chert on the first one. As an archaeologist I've seen gobs of chert -- it was a popular choice for stone tools, and a slabby, tabular piece like that with a crusty rind like that and a hard glossy interior just screams chert to me. Jasper is usually considered a type of chert in my circles but IANAG.

also nthing an unusual sandstone or siltstone concretion for the second one.
posted by Rumple at 11:19 PM on November 17, 2009

PS I have a whole box of concretions in my lab, labelled "naturefacts", which I use to puzzle students with on archaeology practicums. Nothing would surprise me in the world of concretions: I have a perfect turtle, a seal, a banana, a human face, and three or four phalluses.
posted by Rumple at 11:21 PM on November 17, 2009

Ancient golf-ball hole-in-one rack?
posted by gjc at 5:29 AM on November 18, 2009

Definitely chert for #1 and common sandstone for #2. IAMNAG, but I have taken Engineering Geology and it seems like any geologist would be able to easily ID these. They are both obviously sedimentary.
posted by JJ86 at 6:21 AM on November 18, 2009

I agree that the second one is some really awesome perfectly round concretions in a sandstone. Concretions are the same material as the rest of the rock except with pore spaces between the grains filled with a another mineral known as the cement so its not odd that they look like the same material as the rest of the rock.

The first one looks very much like some kind of chert to me (which flint is) but it could be some kind of crazy carbonate. see if you can scratch that polished surace with a nail and if you cant I am pretty happy calling it chert.

Nice rocks. I am super jealous of the concretions.

Agreed. Chert and concretions. I've never seen perfectly spheroidal ones, but I've seen ones that are close. That's a pretty cool chert chunk too.

IAMAGeologist.
posted by Big_B at 9:05 AM on November 18, 2009