Back in December, Diego emailed me pictures of an interesting looking, rounded rock that has some lighter markings on it that look like pictures.

Diego wrote:

Hello Evelyn,

My name is Diego, and since you like to study rocks, I do have one for you! I was told it was sediment that created the pictures on the rock.

Is it possible? What would you do if you have found this rock? Any ideas would be most helpful. And do what you want with the pics.

Enjoy the Holiday and the New Year. I am in no hurry for the answer, and if you need more pics, will send. Or if you like I will send it to you as long as you don’t break it! If this helps; at 60x it looks to be sand turned into glass.

Thanks for your time.

Sincerely,

Diego

I wrote back to Diego to ask him for a little more information about his rock. Specifically, I wanted to know where he found the rock in order to obtain a little bit of regional context. Diego wrote back that he found the rock in Winona, Kansas. He says that the rock came from the bottom of a ~25 ft deep hole and that he picked up the rock when he was helping a friend repair some sewage lines. Diego has had the rock for 7-8 years and is fascinated by the light-colored patterns on the rock. He’s shown the rock to people over the years, and they’ve seen many things in the patterns, such as:

Clouds, face, sun/moon, cave/house/mountain, bird, worm and the last thing on the bottom the kids actually got that one. One said it was an angel, another said it was a dancer, and one boy said it was someone riding a dog! Who knows though, all say it looks like a bear.

Diego is curious if some geologists can help identify his rock. I thought I’d write down my ideas here and then perhaps some other geologists can write down their ideas in the comments.

Here are some more pictures of the rock:

Diego, I appreciate you willingness to send me the rock, but I don’t think that will be necessary. However, I’m afraid that in order to truly identify your rock it may be necessary to break it. There appears to be a weathering rind on the rock, and in all honesty I’m not sure I can fully identify it without seeing the less-weathered interior. If you’re willing to break even a small piece off the rock to expose the interior, that would be very helpful!

With the pictures I have so far, though, let me try to do some basic identification. As always, let me start with the basic identification questions. Then I’ll move on to a few more specific questions.

Is the rock natural or manmade?
Based on the morphology and weathering of the rock, I’d say that this is a natural rock. The rounding of the rock gives me a clue that this rock has spent some time in a stream or other body of water, which eroded and smoothed the rock.

Is the rock igneous, sedimentary, or metamorphic?
Because of the brown weathering rind, I’m afraid I cannot definitely identify the rock type. Many different types of rocks develop similar-looking brown weathering rinds. If I had to guess, however, I’d say that this is either a fine-grained igneous rock such as basalt or some type of sedimentary rock.

So, what is the rock?
The rock is a rounded cobble which spent some time in a stream or other body of water. The rock has also clearly been weathered producing a brown weathering rind. The rock is probably igneous or sedimentary.

What are those strange markings on the rock?
I’m actually not completely sure. Diego, perhaps you could send through some slightly higher resolution photographs which are close-ups of those lighter patches? Those markings could be places where the lighter interior of the rock is being revealed or, perhaps, they are places where a secondary infill has been deposited. One interesting thing I noticed is that the lighter markings are absent from at least one side of the rock.

Why do those strange markings look like pictures?
I believe that those markings are natural rather than manmade. The fact that the markings look like pictures is probably just a result of the pattern-seeking nature of the human brain. The human brain looks for patterns everywhere– that’s why people see, for instance, an image of the Virgin Mary in a grilled cheese sandwich. Personally, on quiet days I like looking for patterns in the clouds– it’s remarkable what patterns the human mind can detect in natural objects such as clouds.

To summarize, this mystery rock is still somewhat of a mystery. We know that the rock is a cobble which spent some time being rounded and weathered in a stream or other body of water. The rock also appears to have a brown weathering rind that obscures identification of the specific rock type. The light-colored markings on the rock are most likely natural, but I’d have to see a higher-resolution photograph or the rock in person in order to identify them further.

To help further identification of the rock, here are some things which Diego can do:
1. Break off a small piece of the rock or saw the rock in half, if he’s willing. This will be the best way to further identify the rock.
2. Take some higher resolution pictures of the rock, particularly of the light-colored areas.
3. Show the rock to a geologist in person. Are there any geologists in the Kansas area willing to look at Diego’s rock?

That’s all I have to say. Diego, if you’re willing to break your rock and then send me pictures– or even just send higher-resolution pictures of the rock’s surface– I’d be happy to devote another post to this mystery rock.

What do other geologist think about this rock? Also, if you want to see any of my previous mystery rock posts, you can find them here.

Time for another mystery rock! This one was sent to me by Jess back in July. Jess, I’m sorry this took me so long to post. I blame my thesis… and wedding busyness… and also the fact that this rock actually has me stumped. I’m afraid that I can’t provide too much more insight into what this rock might be, other than that I think it’s sedimentary. Not very helpful, I know. I’m hoping that other geologists in the geoblogosphere can provide some more useful insight.

Here’s Jess’s original message to me:

Hello Evelyn!

I would SO appreciate your expertise in figuring out what kind of rock I have. My husband and I just recently took a trip to Traverse City, MI, and I found this rock about a mile from the shore in Lake Michigan. We took it to a few rock shops, and one owner said he had only seen a couple in the past, and that is was a jasper-hematite “blend”. He also said that one man brought one in, wanting to find out if he could tumble it, and because it is so hard, the shop owner told him it would take about 9 months to tumble. The man cut his rock in half with a wet-saw, which reportedly took about 6 hrs. to cut all the way through. The pattern on the outside is apparently the same throughout the rock. I’m barely able to scratch it with a roofer’s nail, and despite thorough cleanings it still smells faintly of sulfur (?). If there are any other details you need, please let me know! I would LOVE to know what this rock is, if you could help me out. Rock-hounding is an absolute passion of mine… determining what it is I’ve found is half the fun for me, but this time I’m stumped! Thank you either way:)

-Jess

Well, I’m stumped, too, Jess! Hopefully some other geologists can help us out.

Here are several more pictures of the mystery rock:

Any ideas, anyone?

*******************

More Mystery Rocks:

Mystery Rock #1

Mystery Rock #2

Mystery Rock #3

Back in May I posted about a Mystery Rock and then a Mystery Rock #2. Pictures of these two mystery rocks were sent in by blog readers. I had a great time thinking about these two mystery rocks and helping with their identification, and there was some great discussion among geologists and non-geologists in the comments.

For awhile I’ve been meaning to post Mystery Rock #3. A couple of months ago, my good friends Jennifer & Jeff, who live in Chicago, sent me some pictures of their kitchen countertop. The mystery of this rock actually isn’t the identification of the rock. The countertop is granite, probably a true granite. I have a little bit of trouble estimating the mineral proportions from the pictures (of polished rock! the horror… all the distinctive cleavage is gone!), but the countertop is definitely a true granite or a close granitoid relative.

For those of you who are not familiar with the classification of granites and closely-related rocks, igneous rocks with less than 90% mafic minerals (such as olivine and pyroxene) are classified using the QAPF Diagram. The letters in the acronym stand for Quartz, Alkali Feldspar, Plagioclase, and Feldspathoid, which are minerals or groups of minerals. For igneous rocks with large crystals that can easily be seen in hand sample (these are called plutonic rocks), the plutonic QAPF diagram is used to determine the rock name:

Granites and granitoids plot in the top triangle on the diamond-shaped diagram. A true granite contains 20-60% quartz and about equal proportions of plagioclase and K-feldspar. If a granitoid rock contains more plagioclase than K-feldspar, then it is called a “monzogranite.” Similarly, if a granite contains more K-feldspar than plagioclase, then it is called a “syenogranite.” Granitoid rocks that contain more than 65% plagioclase are technically “granodiorites” while granitoids that contain more than 90% K-feldspar are technically “Alkali feldspar granites.” In addition to quartz, plagioclase, and K-feldspar, granites also usually contain mica (biotite or muscovite or sometimes both) and hornblende. Apatite, zircon, titanite, and magnetite are commonly present in small amounts.

If you want to learn  more about the use of classification diagrams for igneous rocks, here’s a great website.

Here’s a picture of Jennifer & Jeff’s countertop, with some minerals labeled:

An aside before I go on to the mystery: many kitchen countertops are actually not granite. For example, countertops are often made of rocks such as granodiorite, quartzite, rhyolite, travertine, marble, soapstone, or gneiss. Many countertops are manmade– they are made of pieces of rocks, often quartz-rich, that are put together in a manner designed to be pretty and also nonporous. As a geologist, I sometimes run into trouble when I visit someone’s house and compliment the rock countertops. Fortunately, many of my friends are fellow geologists or scientists (or are poor students who can’t afford nice countertops), but sometimes the conversation goes something like this:

Me: Wow! Those are some beautiful kitchen countertops you have.

Host: Thanks! I’ve always wanted granite countertops. When we built this house, we decided we just had to have them, even though it was quite expensive.

Me: Granite? These countertops aren’t granite. But they’re gneiss.

Host, slightly taken aback: What do you mean they aren’t granite? They must be granite.

Me: Oh, they’re not granite. But they are gneiss, which is…

Host: Nice? Just nice? Henry, did you hear this? She says our countertops aren’t real granite!

Host’s Husband, Henry: Well, of course they’re granite. I bloody well paid enough for them.

Me: Please, let me explain. I mean the rock type gneiss, which I actually think can be prettier for countertops than granite.

Henry: Ha ha! Oh, I’ve heard of gneiss. You geologists have gneiss schist, don’t you know?

….awkward digression into geology puns…

Me, desperately attempting to change the conversation: And you have such beautiful cabinets, too! I love the cherry color.

Host: Yes, but you scientists are probably going to tell  me they’re not real wood…

I bet many a geologist has had a variation of the above conversation. Anyway, Jennifer & Jeff do have a real granite countertop– quite a pretty one as well!

Here’s a few more pictures of the countertop:

And here’s a picture illustrating the rock mystery:

Jennifer & Jeff noticed that a small magnet was attracted to the edge of their countertop– but only to the dark portions of the rock. The dark portions are likely hornblende and/or biotite (it’s a little difficult for me to tell from the photographs of the polished surface). These two minerals are generally not magnetic, so my guess is that these dark minerals contain inclusions of a mangetic minerals such as magnetite, which is commonly found in granites in trace amounts.

Do any other geologists have insight into this countertop mystery? Does anyone else have a magnetic granite countertop?

Earlier this month I posted pictures of a mystery rock that were sent to me by one of my blog readers. On Friday I received an email from someone who had been directed to my blog from the Skeptics Guide to the Universe Forum. A friend of this person had found an interesting rock in a rock wall in El Paso, Texas and wanted to know if anyone could identify this rock. Below are some pictures of beautiful Mystery Rock #2:

Mystery Rock #2, Photo 1.

Mystery Rock #2, Photo 2.

Mystery Rock #2, Photo 3.

Mystery Rock #2, Photo 4.

This rock is a little bit easier to identify than the first mystery rock. Again, though, geologists do find it more challenging to identify rocks from pictures (rather than in person) and from a rock wall (rather than an outcrop).

Let’s go through our identification questions:

-Is the rock man-made or natural?
This is a natural rock.

-Is the rock igneous, sedimentary, or metamorphic?
This is an igneous rock.

-What type of igneous rock is this?
This appears to be a porphyritic igneous rock with phenocrysts of feldspar in a very fine-grained or possibly glassy matrix.

Let me explain some of the terms in the rock description for those of you who are not geologists. Igneous rock literally means “fire rock” and is a rock that solidified from a partially or completely molten state. Igneous rocks either slowly crystallize deep in the Earth or rapidly crystallize on Earth’s surface from molten rock erupted by volcanoes.

Porphyritic is a term used to describe igneous rocks that have a distinct size difference in crystals. In the case of the rock above, there are phenocrysts (large crystals) in a matrix of much smaller crystals. It is a little difficult to tell from the photos, but the matrix seems to be very fine-grained, possibly even glassy. The matrix either consists of tiny crystals that are too small to see in the photos or is glassy, meaning that there are no defined crystals but rather amorphous, unordered solidification.

How to porphyritic igneous rocks form? Well, they generally form when magma that has been slowly cooling for a long time, possibly in a magma chamber, is suddenly erupted to Earth’s surface. Deeper in the Earth where magmas can cool more slowly, large crystals have time to form and grow. Those phenocrysts are quite large (they are so large I’d even venture calling them “megacrysts”), so they had to form through slow growth over a very long time in a magma chamber or similar environment deep in the Earth. Magma takes while to crystallize completely, so sometimes partially-crystallized magmas are brought to Earth’s surface and erupted as lavas. When these partially-crystallized lavas are erupted, the rest of the molten rock cools quickly, and there is no time for large crystals to form.

Minerals crystallize out of magma in a certain order, following something called Bowen’s Reaction Series. In certain magmas, feldspar is one of the first minerals to crystallize out of a magma. My guess is that the large crystals in Mystery Rock #2 are feldspar. It’s always difficult to identify minerals from pictures, but the crystals look like feldspar to me– they are the proper shape, and they exhibit characteristic feldspar twinning. So, my guess is that Mystery Rock #2 formed when magma first spent some time deeper in the Earth and crystallized large feldspar crystals. Then, that magma containing the feldspars was brought to Earth’s surface and erupted. The rest of the molten rock cooled so quickly that there was no time for large crystals to form. Instead, the lava rapidly solidified into very tiny crystals or perhaps even glass, which forms if the cooling is very, very rapid. If the rest of molten rock had had time to slowly cool deeper in the Earth, perhaps the rock would have been something like a granite rather than a porphyritic igneous rock with feldspar phenocrysts.

The person who found this rock also mentioned that the large crystals have a very pretty green hue to them. Feldspar is generally white, brown, gray, or pink, so perhaps I’m missing something, but I believe the green hue is just from alteration of the feldspar crystals.

Other geologists– what do you think? Please weigh in on the identification of Mystery Rock #2.

Finally, to the owner of this rock– this is a truly gorgeous rock. If you ever decide you want to rid yourself of this rock, please give it to a geologist or other rock-lover.

Mystery Rock, Photo 1.

A few weeks ago a blog reader named Bob wrote me to ask if I could help identify a “mystery rock” in his backyard. Bob sent me several photos of the rock and wrote:

Hi Evelyn,

I’m reasonably sure I’m wasting your valuable time with these “silly”
rock pictures, but I’ve been searching the web for similar pictures or
descriptions with no real satisfaction.

I am a computer tech by trade and know nothing about rocks.

I found this one unique looking rock on my property in Bergen County NJ.
It was one of I’m sure tens of thousands in a large rock wall that is at
least more than 75 years old. (It could be much older.)
I have seen none else that look like this one so far.

To keep this email short, here is my question:
Should I be more curious about this rock or just place it in a garden
bed for decoration?
 
Thank you very much for humoring me on this.
Bob.

Well, as a geologist I like nothing better than puzzling over an interesting rock– so, Bob, you’re not wasting my time at all!

My fiance Jackie (a geologist who currently works for AuruMar) and I took a close look at all of the photographs Bob sent of this mystery rock, and we’ve tried to identify it to the best of our ability. I thought I’d write up a post with some pictures of the mystery rock and our attempt to identify the rock.  If any other geologists want to weigh in on the identification of this rock, please do so in the comments.

Before I proceed with the pictures and our identification, I want to make two comments:

-First, identification of this rock is limited by two factors:
1. We are identifying this rock from pictures, which is not ideal. Geologists like to see rocks in person so that they can squint at them with hand lenses, scratch them with their fingernails, feel their density, break them open with hammers, and– on occasion– taste them.
2. This rock was in a stone wall, not in situ. Identifying rocks that are no longer in situ is always more challenging because of the lack of geological context.

-Second, Bob sent some good pictures in his first email, but when I wrote back to him I asked him to do two things: 1. If he was willing, break off a piece of the rock so that I could see the less-weathered interior, and 2. Try pouring some weak acid on the rock to see if it fizzed. You can see pictures with broken surfaces below. Bob informed me that when he poured muriatic (aka hydrochloric) acid on the mystery rock, “the muriatic acid from the hardware store caused my patio pavers to fizz up!… but not the ‘odd rock’.” I asked Bob to pour acid on the rock because carbonate rocks (such as limestone) fizz when acid is poured on them. Since the rock did not fizz, it does not have a significant carbonate component.
 
Here are some more pictures of Bob’s mystery rock (click to enlarge):

Mystery Rock, Photo 2.

Mystery Rock, Photo 3.

Rock wall where the mystery rock was found.

Mystery Rock, Photo 4.

Mystery Rock, Photo 5.

Mystery Rock, Photo 6.

Mystery Rock, Photo 7.

Mystery Rock, Photo 8.

Mystery Rock, Photo 9.

Mystery Rock, Photo 10.

Here is our attempt at identifying this mystery rock:

Identification: A poorly-sorted sandstone containing some angular rip-up clasts and with possible animal burrows (edited to add: probably not, but that would have been cool). We suspect this rock formed in a fluvial (rather than marine) environment. This rock likely formed in a high-energy stream or river and was also rounded in this fluvial environment. Edited to add: Another possibility is that this is a glacial erratic (a rock picked up by a glacier and dropped elsewhere). Perhaps the rounding occurred in the glacier? 

Now, let me explain our reasoning behind the identification:

The first question to answer when identifying a rock is if the rock is natural or man-made. Geologists must be careful not to misidentify anthropogenic materials such as brick, cement, and road tar as natural geological materials. We believe that this is a natural rock, not an anthropogenic material.

The second question to answer when identifying a rock is if the rock is igneous, sedimentary, or metamorphic. This rock is clearly sedimentary as you can see rock fragments and grains of different sizes which have been cemented together.

Okay- so we’ve identified the rock as natural and sedimentary. Now, let’s move on to some of the rock’s features. The rock has larger clasts (some of which are angular) of at least a couple of different rock types (maybe the one angular clast is a red mudstone?) set in a finer-grained (but still fairly coarse) matrix that looks (to us) to be mostly quartz grains, which would make the matrix of this rock a sandstone. Because of the rock’s sandstone matrix, we don’t think the angular clasts indicate that this rock is a breccia. Rather, we think these angular clasts are something called “rip-up clasts”– basically, fragments of rock that are ripped up in a high-energy sedimentary environment. This environment could be a high-energy stream or possibly a marine environment (for example, turbidites often have such rip-up clasts in them). Since the rock is well-rounded, we suspect this rock came from a river or streambed, meaning that it may have originally formed in a streambed as well.

Finally, we wanted to comment on the rounded holes located in the rock. These could be cavities which used to contain large, rounded clasts that have now weathered out of the rock. However, we think these large, deep, round holes could be some type of animal burrow.

That’s our attempt at identifying this rock– any thoughts from other geologists?

I hope this helps you, Bob! Thanks for sending so many great pictures of this mystery rock. Finally, to answer your original question: please do place the rock in your garden bed, but be sure to point it out to people and say, “did you know that’s a poorly-sorted sandstone?”