Can anyone tell me what this might be. We were on the south coast and my children found this rock?? It was already split in 2 but fits nicely together. There appears to be some crystals/crystalisation in the middle. There is bulge on one side. I guess it weighs the same as a rock of that size and as the photos show it fits on a chopping board. Any suggestions.
In summary, I think it is a small vein, probably of calcite (which can be of different colours), along one side of which the rock has cracked open - so you are seeing the vein 'flat-on' rather than the usual 'edge-on' way. All veins have limited extents, and some are quite small. In your case the aerial extent is roughly cirulcar in shape, maybe making it look like it might be significant.
If you don't mind damaging the specimen, you could try scraping some of the crystals away, to check they are actually in a thin layer (vein).