Rock Identification

[Concrete]

Can you tell me whether this rock is natural or made by human (for example a kind of pressed rock mixture produced for building works)?

I have found it on the seaside in North Berwick, Scotland, along with many similar rocks (the pattern is the same) but with different colours and I need to be aware of what I'm going to collect.

Thanks 

 

[jasong]

Natural. Looks like a basalt porphyry. It came from a lava flow, it's vesicular. The white crystals (fancy geologic term: phenocrysts) are probably some kind of feldspar. Basalt will vary in color, reds/browns, etc. 

So there is probably some old lava flows somewhere nearby from which this originated, or being on the ocean it may have been discarded ship ballast.