GeologyRocksThames pebble
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I'm going to make a few
Mon, 07/30/2012 - 20:41I'm going to make a few amature guesses here, so please wait for Gus or John to chime in with more reliable answers.
It appears to be agate, perhaps a badly weathered geode (thunder egg)? I'm also going to guess that the pale yellow crystals are maybe chalcedony or just plain old quartz.
Off the record, it looks like an avocado minus the seed 
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Wow a thunder egg?! Never
Mon, 07/30/2012 - 20:51Wow a thunder egg?! Never heard of one of those before although after asking google i can see that they look lovely.
It is a lot like an avocado without the stone actually, perhaps a Devonian period avocado fossil? Probably not.
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I'm fairly confident it's a
Tue, 07/31/2012 - 01:51I'm fairly confident it's a pyrite nodule, from your description and the first picture, I can't get any of the others to load on my slow connection at work.
Gus and John should be able to either confirm this, or come up with somethign completely different 
I'm not to good with the rock stuff, I much prefer fossils!
Leah
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I can see why you think it
Tue, 07/31/2012 - 11:08I can see why you think it might be pyrite as there appears to be a metallic lustre but there's also a conchoidal fracture which you wouldn't normally associate with pyrite. Therefore it's either flint (which I'm not 100% convinced about) or it's a piece of man-made debris which could be a fragment of rivet or something similar (the powdery yellow deposit being an iron oxide). The best way of telling is by comparing weights. if it feels about the same weight as a "normal" pebble it's probably flint, if it feels very heavy compared to its size it's probably iron.
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I'd agree with
Wed, 08/01/2012 - 16:12I'd agree with Gus. Pyrite would have rusted away anyway. Certainly looks like flint. And probably from around the Reading area where the Thames flows through various gravel beds.
You should put an everyday object - such as a coin - next to it as a scale when photographing
John
“Civilisation exists by geological consent, subject to change without notice.” - Will Durant
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Thanks all, It's not
Wed, 08/01/2012 - 20:25Thanks all,
It's not metallic looking in the flesh, as it were, the black shell would almost plasticy if it weren't cold to touch and sounds like rock if tapped. It feels about the right weight for a pebble so I think it's probably flint, as you said. It's nice though, very tactile.
I'd never visited this site before I posted but I spent an hour looking through previous posts, I think I might be hooked!
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Thames pebble
Submitted by Minnie on Mon, 07/30/2012 - 18:48.Hi all,
I found this little rock on the banks of the Thames river at the weekend and wondered if you could tell me what it is and how it looks the way it does.
It's a small egg spherical rock, black and pitted on the outside with little marks which look like digs from fingernails. The inside (really it's only half a sphere) is orange-brown and looks to my uneducated eye like flint due to the glass-like chips. The surface of the 'cut' surface is very smooth and shiny but the rock has a round hole inside which looks sandy. There's a surface of pale yellows crystals inside.
I'd be interested to hear what the crystals might be too.
Can anyone help me out? I've googled all kinds of things but 'black stone shiny stuff' doesn't return any helpful results. Surprising!
http://photobucket.com/albums/ii510/Minbee85 Thanks all.