Found this near Cardiff on the coast. Lots of devil's toe nails and a few ammonites and crinoids nearby. Not sure if this is part of a plant or an animal. Looked for ages and couldn't find an image which looks like it. Any help really appreciated. Thanks in advance for your time.
Although your second image looks like it could be a fossil I think you have a partial septarian nodule, it is were a rock has let water through small cracks and left behind a mineral deposit that has built up over millions of years.
Your specimen is heavily water worn as to be expected comming from a beach environment and similar to what i find on the Holderness coast.
I split a septarian nodule tonight and found a layer of mineral that I took a couple of images of and attached below.
Sorry about the quality of the images but I hope you can tell from image one (the whole nodule) were all the lines of mineral are running all over the rock.
If it is a fossil it could be a Pinna sp bivalve.
Tabfish, I see where you're coming from with vein and Pinna. I'm not convinced of either, but I don't have a better suggestion.
I can't quite convince myself it is fossil, so I lean towards a vein. When a rock breaks along a vein, it can break on one side of the vein, leaving a veneer of the host rock attached - which may be what we are seeing in your specimen as the grey, granular surface. What bugs me, is that it is not nearby as smoothly eroded as the rest of the rock. OK, so maybe the rock is a grey limestone and the vein is quartz. Still I'm not 100% happy with that; can't put my finger on why. And my mind wanders to the idea of a cuttle bone (but it can't be that because a cuttle bone is lens-shaped in cross section, not a layer like yours).
The small holes (IMG_6175.JPG) are recent borings, eg. by small molluscs.
If somebody else doesn't give you an ID, you might need to send it to the Angela Marmont Centre for somebody to take a close-up look.
Thanks for looking, and your suggestions, still not sure what it is. How do i send the images to the Angela Marmont Centre?
I meant for you to (potentially) send the specimen to the AMC.
In the first instance, phone them (number in the link). They may point a geologist to look at this discussion. If that proves inconclusive, they should be able to guide you, perhaps to sending the specimen to them, or bringing it in person.