SBB[21] / rmf - webbed mineral [Explored]

This was found below the normal water line at Cow Green [currently the level is well down] in a pile of similar stones.

The substrate is smooth surfaced, but appears crystalline, mainly white and grey with some darker veins and patches. The "webbing" appears to be granular, almost sandy in texture and colour, but as it is much harder and resistant - perhaps by the cement between the grains, it is raised several mm in most places above the substrate.

My limited knowledge of geology hasn't yet produced an ID for this specimen. Further research required.