I found this piece at [Battery-point, Portishead, UK], at a site for fossil hunting.
It is smooth one side and porous on the other. It also has one side with beautiful colours shining on the surface.
I’ve attempted to take pics which show it but if I’ve missed something please ask. It’s hard material, and I’ve not washed it properly but each time I give it a quick clean more colour shows. Added some pictures in better light