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I found this curious rock in the Brecon Beacons, a mountainous range in South Wales. The regular dotted pattern stood out from the plan slate and gravel in the area and I'm not sure if these are fossils or quartz?

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The white dots run through the whole stone.

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This particular rock was part of a gravel pathway and shallow quarry where the gravel was excavated from. This dotted pattern stone seemed quite abundant there.

The dark rock is fairly unremarkable and can be scratched with a coin. I believe there are quite a few slate quarries in the area and it does feel a bit like slate or similar.

I can't be certain it's the same but in the quarry it looked like the dots slightly extruded from some of the rocks (my apologies for the photo quality)

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Does anyone know if this is fossil or quartz or how this pattern could of formed? I'm really curious.

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Pretty sure you have a fossil coral in limestone, dilute HCL acid test will confirm calcium carbonate. I have found similar fossils, at least in form, in the front ranges of the Rock Mountains of BC. It seems that the UK has a number of corals, the commercial site here has images of a fossil-bearing rock that looks similar to yours, though this example is from Ireland. Hopefully someone with more knowledge on the fossils of your area will give you a better answer.

If you have a number of samples, try placing them in dilute acid. Sometimes the fossil material is more resistant to the acid and the result is that the fossils get etched out of the host material. Careful though too much acid can make very fragile fossils or totally destroy your sample. I have used full strength muriatic acid but it is really too strong.

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  • $\begingroup$ Fossilised coral, yes I can totally believe that. It does look identical to the link you shared. Crazy to think that area was once a coral bed. Thank you very much for sharing and taking the time to answer. $\endgroup$ – user2992260 Apr 29 at 17:17

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