This interesting pebble caught my eye the other day when I was wandering around on an Anglesey beach in Wales, UK but I haven't a clue what sort of rock it is and what has caused the little holes in it.

Parts of it are smooth while others are covered in holes (see below)

Any suggestions would be greatly appreciated - I am just really curious :)

enter image description here

  • Maybe this is consisted of shells of dead sea life.
    – Vorac
    Apr 9, 2014 at 12:30
  • 6
    This will be an excellent question for the new Earth Science SE which should be up and running very soon. Apr 9, 2014 at 17:08
  • @ChrisMueller feel free to move it to there (if that is possible) I want as many people as possible to have an input on this - I have really been intrigued by all the answers I have received so far
    – user3127
    Apr 10, 2014 at 8:41

1 Answer 1


That looks like fossilised coral (possibly a sponge?), the outside is smooth because it's been worn down by the erosion of the sea.

There is quite a lot of fossilised corral on the north Wales coast, it was once a shallow, tranquil, tropical Carboniferous sea

The Carboniferous period from wiki:

The Carboniferous is a geologic period and system that extends from the end of the Devonian Period, about 358.9 ± 0.4 million years ago, to the beginning of the Permian Period, about 298.9 ± 0.15 Ma

Making it about 300,000,000 years old!

It's a very nice specimen BTW

  • forgot to ask, does it have a proper (scientific) name? or is it simply called "a fossilised coral" ? Would it be possible to estimate its age? (I guess this has to be done by taking it to an expert in the field)?
    – user3127
    Apr 9, 2014 at 14:12
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    I don't know. I would guess it would be simply called "a fossil". I am not a geologist, though I do have geologist friends (I'll ask them). I've added details from Wikipedia on the period, I don't understand the age notation they're using, anyone?
    – user2766
    Apr 9, 2014 at 14:21
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    @Bo5ao, There are several processes which result in fossils; some result in rocks while other don't (technically). Without knowing what kind of fossil it is, it's impossible to say whether it counts as a rock!
    – Brian S
    Apr 9, 2014 at 14:56
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    @Liam, Are you referring to the "298.9 ± 0.15 Ma" bit? Ma is short for Megaannum in this context, which is 1 million years.
    – Brian S
    Apr 9, 2014 at 14:58
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    @Liam, "±" is the "plus or minus" symbol. "298.9 ± 0.15" means "298.9 - 0.15 or 298.9 + 0.15", though in most usages it means "anywhere from 298.9 - 0.15 to 298.9 + 0.15" -- it's giving an approximate range on the possible values.
    – Brian S
    Apr 9, 2014 at 15:05

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