Hi. I found this unusual rock today in a pile of pebbles in my grandparents garden. Could anyone ID it? I also found 11 Gryphaea but they were in poor condition. I think they were dredged from the North Sea. Two of them were in decent condition, but they had a few chips around the edges.
It is either a marcasite nodule or a lump of something that was once iron but is now thickly corroded to something close to rust. You might want to break it open...
If it is marcasite, it should show a radial pattern of golden needlish crystals.
If it was iron, it will be very disappointing in comparison!