It is a bit difficult to tell from this photo, is there any chance you could add another image maybe of the other side or from another angle? Also can you tell us anything about the weight or texture (there appear to be little vesicles/bubbles along one side - is that the case)?
I have taken a few more pictures, including underneath and a few more angles which I hope might help.
It does seem to have 'bubbles' - particularly showing in the crevices on the top.
It is very triangular in shape and one of the sides is very straight, so I wondered whether it had been cut at some point.
We don't have any scales to weigh it, but with the help of a metal ruler as a balance and some mini Babybell cheeses (yes, we really did this), we estimate it to be about 38 grammes.
It's sides measure approx 3.9cm, 4cm and 4.2cm and it is approx 1.6cm deep.
Thanks for your help Hazel,
Ann (Cluffy) and John
Unfortuntaely the pictures don't look very good when uploaded. The originals are better quality than when uploaded, not sure if there's a better way to upload them or send them to you.
I think the rock you have found is the slag from a blast furnace.
I do not know how it got to Spurn, it could have been the ballast in a ship, road making materials or simply long shore drift.