It is not ringing any bells...
Please could you determine its actual density: weigh it then calculate its volume by immersion in water (it looks to be fairly unabsorbent); for preference, use grammes and cc (=ml).
- Does it 'ring' (resonate) when you hit it with something hard?
- If you knock a corner off, is the fresh surface bright (perhaps metallic-looking) or dull? (I understand if you don't want to break it.)
This is an iron sulphide nodule (marcasite / pyrite). I often find them in chalk. I have one that looks surprisingly like a poo!
Quagga - Aye!
Flox - I hope you're glad you broke it now!
Sometimes you break/cut unknown/dubious objects open and it tells you next to nothing. Nice to see a case of where it is a real eye opener.