I found this ball in a rock pool at Burling gap, immediately thought it was an old cannon ball due to size, colour and weight and site! but googling rusty old cannon balls they appear to rust in layers and this has a very particular strucure to it, also it is not in the slightest magnetic.
I guess that that its round shape may be due to being rolled around in a rock pool by the sea, but all the other rocks in that area are chalk or flint. The weight and intricate structure are a mystery to me - any ideas? weighs nearly a kilo for a stone just bigger than a tennis ball thats heavy!
It's something called a pyrite nodule - they're mineral accumulations that form in rock, particularly the Chalk as found in Birling Gap. Pyrite is also known as fool's gold. Unfortunately pyrite does break down in a process known as pyrite decay, and this one has already started. I'll post some more information when I get to work.
I've now attached some info sheets that go into more detail.