Someone started a thread not long back on the same topic, but I have failed to find it; so I have to start a new one.
Does anyone have a view on how this formed? The highlighter is for scale.
I am sure that the rock is sandstone, but I might be wrong. It looks like someone made a rubbish pot, then filled it with a softer rock.

Water worn brick?

+1 to Shotlighters ID of very abraded brick as my first guess, or failing that it's a sherd of pottery, potentially very old pottery! Either way, it's man-made ceramic of some sort.

Top photo has a profile face.

Did you mean this thread?:
https://ukcaving.com/board/index.php?topic=25308

That was the one.
It doesn't look like a water-worn brick, but who knows?

TOR does like his rocks!

I have consulted a geologist friend of mie who says that it looks like an "ironstone concretion around a sandstone core. Iron ore (probably siderite) was deposited along joints in the sandstone, effectively surrounding the inner block of sandstone".

When the lockdown is over I'll show him the actual stone and see what he says then. Wait for the update.

I think this may be an example of the situation, though I'm no geologist - but it seems as though siderite (if that's what it is) has been deposited via a joint in the stone, but has then created essentially a part-complete vein in what must have been a small fault, as the mineralisation extends across the working face in a relatively thick coating, almost like calcite would in limestone. As I mentioned, I'm no geologist, but I'm interested. This is in Millstone Edge quarry near Hathersage.

Got plenty of nice shots of ironstone nodules in shale/sandstone beds, with an adjacent coal seam too, though from a different site.