This Friday I have decided to give you a mystery object that I have to deal with on a regular basis. Something that I get more enquiries about than anything else. Do you know what this is?
Your only clues are that they come in a range of sizes and they are often found on the South Coast of England by walkers.
As usual, answers below – I will do my best to respond to questions, but I have a submission deadline looming for corrections on a paper, so I may be a bit tied up. Good luck!
Zygoma 
I would say it is a pebble from Chesil Beach, and if I looked up the size I could tell you exactly where, since they are size-sorted. However, you’d think anyone would be able to identify a pebble.
The south coast is famous for it’s fossils so I wondering whether it is something from a fossil. The shape looks quite rounded, like the ball in a ball and socket joint but I don’t think it’s that.
Is it a gullet stone from some large, extinct marine animal? (I believe various bird like creatures swallow stones in order to get some chewing action, this one is quite big) Implies the animal is vegetarian.
(Image doesn’t appear in blog for me but does when I click on it.)
Tea break, so I have time for a couple of quick replies.
Not a pebble from Chesil (I remember doing that size sorting experiment there for my A-Level geography many years ago).
Also not a gullet stone (or gastrolith) although it is similar in size and shape to Iguanodon gastroliths we have in the collection – the surface of gastroliths is very polished though.
Not sure what’s wrong with the image – it shows up fine for me – I’ll see if I can do anything about it when I get the chance…
My first guess on looking at it was “Cannon Ball”. The clues seem to fit, so I am going to submit it. I could imagine a cannon ball getting interest and also them turning up on the south coast (marauding French Dutch and Spanish).
Interestingly “Cannon Ball” is a colloquial name for one of these, but it normally refers to one made of iron. This one is not metallic (and it’s not an actual cannon ball).
Duh, brain’s going, can’t remember the name. But nodules of stone that when split have mineral crystals inside
I think geode is the word you’re looking for – however this is not a geode.
Although you’re doing well.
Yes, I remembered the word is geode just as the tube dived into the tunnel. And heyho, no chance to get back until here on the Northern line again.
I agree, you get them because people think they’re fossilised eggs and if not a geode then a flint nodule – and nodule is, I believe, the correct term here. I should have just said it’s a nodule.
How about a flint nodule? Right location and it is geological.
w00t! Pretty close!
I think it’s definitely a conretion of some sort – at first I thought it wasn’t flint, but I looked again and saw that that whie is just the outer dirt chipped off – the black section shoul start a couple of mm below that.
So I’m going to back SmallCasserole on that. Unusual in that it’s spherical, right?
They are quite frequently fairly spherical – it depends on the nucleating object. Often it will be an echinoid, which are pretty spherical.
They’ll come in claiming its a “dinosaur egg” won’t they. Except its actually a flint nodule. And some of them won’t take no for an answer.
This is by far and away the most accurate answer I could have hoped for! Muchos kudos!
Assuming it is a concretion of some manner, is there anything particularly notable about the nucleating object? Is there a biological or fossil component at the centre?
Have I just actually asked for the answer?
Yes you have just asked for the answer and yes, there is often a biological nucleation point.
Have you found out what that stone is? I have got one of that as well. Shall i slice it see whats gonna come out inside?
When i try to look inside with a torch half of it looks like empty as lighter colour
It could be empty inside. Possibly a geode or it may be that the internal nucleation object dissolved, creating a void.
Here’s the answer for that particular object: https://paoloviscardi.com/2010/01/18/friday-mystery-object-26-answer/
Spheres are a common form in nature, so there are several possibilities!