Very strange rock find...ever seen this before?

[electriceye]

Hello Folks-

Just happened upon this site and love it already, lots of really neat finds!
After browsing for awhile, I remembered I had a strange rock artifact sitting in my garden. I have never seen this before in a rock and thought maybe someone here might know more about what it is.

Basically this is a decent sized rock, weighing approx 8-10 lbs. It appears to be sedimantary based but not soft, like limestone. It is actually pretty hard!

There is a oval shaped hole almost in the middle of it (approx 1.5"x 2"), that looks and feels fairly smooth..until you look at it with a magnifying glass. There are actually rocks deposits on the walls of this hole, and no signs of modern drilling bits or other manmade boring signs.

Without knowing *what* this thing is, I put together a few different theories:

1. Oval shaped hole in rock was formed by rock growing around a human or animal bone that has since washed away (means it's thousands of years old?)
2. Oval shaped hole was ground out or hollowed out by ancient man to hold a torch in a cave wall. (means it's thousands of years old?)
3. Oval shaped hole was slowly ground out by water over thousands of years. (means it's thousands of years old?)
4. Your guess?

I 'm not really sure what the heck this thing is, but it is intrigiung to say the least. Has anyone else here seen anything like this? One of my friends was over and said it could very well be an ancient man artifact and could be worth a chunk of money...or not.

If you get a chance, take a look at the images (if I can upload them here) and see what YOU think this hunk of rock is. Thanks!

 

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[Twitch]

I believe the rock is made out of flint or chert and eroded out of a limestone matrix. It's a sedimentary rock formed from ancient sea deposits. The hole in the middle is a where a softer fossil or weak spot in the rock eroded out. The concept isn't uncommon though the size of this example is uncommon. I have some level of confidence that this is an accurate explanation for your piece. It's a cool find and certainly would have come home with any of us as well.

 

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[AUVnav]

Given the type of rock, and the hole, it looks like its from a boring clam....

 

[electriceye]

AUVnav-

I'm not convinced- The pictures you have above *look* sedimentary for sure, the ones I posted do not. Also note the hole in the mddle of this rock is egg-shaped all the way down, and not symetrical. I don't see any examples of boring clams making egg shaped oval holes. The cross sections above may *look* egg shaped or oval but that is just due to the cross section cuts.

I will have to see if any ancient seas are located where I originally got this, I don't think so, but not sure. Also, with new rock growth growing on the walls of the smooth oval hole, this tells me it was not underwater that long, if it ever was.

 

[PYRATE]

I will have to see if any ancient seas are located where I originally got this, I don't think so, but not sure. Also, with new rock growth growing on the walls of the smooth oval hole, this tells me it was not underwater that long, if it ever was.

Hard to tell without seeing detailed close-up pics of the rock.

Your camera has a super macro mode which allows photos to be taken within 2" of the subject so please add some more pics of the "rock" where there is no patina

Also what do you mean by "new rock growth" and how is it affected by being underwater?

My initial thought is that this is cement (concrete w/o the sand or aggregate)

 

[ibjeepn]

Hmmmm. A petrified donut ? Cool find.

 

[nuggy]

New rock growth in hole is where water containing mineral seeps into cavity, leaving it's mineral load behind around entry site - sites, like mini stalactite. Nuggy

 

[PYRATE]

if mineral deposition occurred, it would be concentrated along the fractures in the rock. I believe the spots which could appear to be mineral deposition are either concretions or (if it is cement) where cement filled small indentations or soft spots in the wood which formed the hole.

I still believe that someone long ago dug a hole, pounded an oval post in the ground and poured cement around it. The post has long since rotted away and the cement was washed or dumped into a river.

I could be wrong - this may an ancient tool for knocking heads or whacking moles, but without toolmarks I'm doubtful. Also, the lack of weathering in the hole and the fractures (which I believe were caused by the swelling of a wood post) lead me to doubt that this broke off from a larger rock without breaking an ancient tubeworm burrow.

 

[electriceye]

PYRATE-

Some excellent theories, all in all. This item however, is def not cement. Also since it is an actual rock, I'm leaning towards the geofact theory now- rock formed around a bone from hundreds of thousands (if not millions of years ago) and the bone has since vanished or washed away. Only way I can explain both the size and shape of the hole. Now that I have looked closer inside the hole, the areas that project outwards could have simply been imperfections or indents in the bone material that later filled with mineral. Either way I'm trying to locate a geologist at the local university to take a look and at least tell me what kind of rock this is for sure. Once we narrow that down we may be able to answer some more questions. Thanks for the replies everyone!

 

[nuggy]

The reason I said branch instead of bone, is that bones are sponge like, a honey comb of tiny holes - in a sedimentary situation the mineral deposits permeate throughout the bone, preserving the calcium and the bone becomes almost part of the rock. This is why dinosaur bones are found in some sedimentary deposits from the right era. A tough hard branch with a slick outer layer resists the water and mineral for many years, even centuries if no oxygen is present to enable it's decomposition. Nuggy

 

[Digs68]

I still say geofact. I've seen enough of them (they are common here in Missouri, the cave state) that they are somewhat of a commonality. It doesn't stop an old rock-hound like me from picking one up and taking it home though. The hard part of the stone (and perhaps your stone as well) are often chert. Here in Missouri it is often a form of Burlington or Mozarkite (Jefferson City chert). Spalls are also common. They can be small like a marble or bigger than a basketball, but perfectly round. You'd swear they had to be man-made, but they're not. The staining in the hole indicates to me that you either found it in a creek or it sat some place where it could hold water, or had water draining through it.
Either way, it is a neat stone to carry home. I'm sure you'll find a good use for it.