The only things I can see on this specimen are a few flat places with concentric circles. It doesn't appear to have any particular texture, although it is not smooth, and seems to be calcareous. I have looked at it under 10x, 25x, 50x and 100x magnification. I've looked through seashore guides and photos of corals, bryozoans and fossils on the web and I can't figure out where else to look. Please let me know if you have any ideas. Thanks.
I found out that this is Maerl, a coralline red alga, possibly in the genus Lithothamnion. I'm still not sure where it is from, but according to Natural England it grows in beds off the southern and western coasts of Britain and Ireland, around the Scottish islands, also in the North Atlantic and in the Mediterranean. The beds are classified as UK Biodiversity Action Plan habitat due to being very slow growing and fragile, and provide nursery grounds for many other marine organisms.