I found the following on the beach in Hayburn Wyke. The block of shale was loose, in the shingle close to the cliffs, on the southern side of the entry point to the beach. At 1st, I thought it was a fish scale, as it looked like black enamel. Only a few mm (at the upper end as seen in the 1st pic) were visible: the 2nd photo shows it after prepping. The shine is water - I found wetting it brought out the striations.I don't think it's a fish scale now, but I'm not sure whether it's a tooth of some sort, or a fragment of a fin spine from something like Hybodus. I've never encountered anything like this before in twenty years, so identification would be much appreciated.
Thanks for your thoughts. It's been so long since I posted the pic I'd forgotten I'd done it. I now suspect it's a piece of wood. When I gently rubbed a piece of paper over the end, it left a black streak. That makes me think it's plant material, as it seems to be carbon.
Bearing in mind what you've send, I'll have another look at it, and let you know.