I found this rock on the foreshore of the tames in london. I piked it up because it was different to anything i have seen in this area before. The rock is made up of many fine layers some glassy and some mat. there is also some iron pyrites. it is very smooth to the touch. There is a curved indent with dark bluish grey bands.
Do you have any thoughts?
It is stone-like because it is stone!
It is a fossil oyster.
The layering, including pearliness can be preserved, and can be 'enhanced' by pyritization.
Your specimen shows only the part towards the hinge. That area can be broad, but it can also be narrow.
Here's an example that shows some of narrowing (and the 'dark bluish grey' you mention)
And here's one closer to yours regarding narrowness
(from near the bottom of http://blog.harborislandoyster.com/2013_01_01_archive.html)