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baz
Hello all, IMG_0031.jpg  I am new to this forum and hope I can get some information on a rock that was found in cornwall. A work colleague recently found this rock whilst gardening and brought it into work, no-ones got any clue to what it is or how it was formed, hoping to get a few bits of info on this mystery. The find is in clay country near St Austell and is not magnetic, the black part is similar to slate and doesn't scratch, any ideas.
Baz IMG_0029.jpg 
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Chalkers
My guess its that it's a worn ironstone concretion although it would help to know where it was found. Ironstone concretions frequently have a concentric banded appearance such as your find does and this is caused by the outward growth of the iron-rich minerals (could be haematite, goethite, siderite or some other mineral) from an internal nucleus. They occur within sedimentary rocks (normally mudstones, siltstones and sandstones) and can form some pretty amazing shapes that can often lead to them getting mistaken for fossils.
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Dirty Pete
A symmetrical ironstone concretion maybe......I think there are iron deposits in the area.
Pete
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baz
Thanks for the replies the stone was found in penwithick just outside St Austell town centre, about a mile or so from wheal martyn a famous clay museum.
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