GeologyRocks

I'm no expert, but it looks like a septarian nodule.

I agree. Septarian nodule it is.

Are they rare? We found it while agate hunting, I've never seen one in all the years. Thanks gang!

Not particularly, no. Quite common in a number of sedimentary environments, especially where there's calcite veins (again, common).

Some nice pictures here. 

Thanks everyone, nice site, I think I will hang around a bit!

I actually sell loads of them because they are "pretty", and I have to say the ones I get in are nice - a really yellow calcite, thick veined, and there is a reaction between the dull buff coloured clay and calcite, which is a rich dark brown, giving a sort of leopard skin  effect.  But they are present in nearly all clays.  I found quite a lot of large, although poor quality, nodules when they were digging out the Lower Lias, putting the by-pass round my village. 

There is no scale to yours, but I sell mine at £4 each and they are about 2 1/2- 3" specimens.   I think I'm quite lucky to get that amount.

 John

“Civilisation exists by geological consent, subject to change without notice.” -  Will Durant