Can anyone tell me what this rock is?
It was found on the rocky beach at 'Glen Boreraig' on the Isle of Skye, Scottland. Definitely water-worn.
It is 9.5 cm x 4.5 cm with inner darker part 4.5 cm x 2 cm.
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It's quite difficult to tell without fresh surfaces, but it looks like a small intrusion of probably basalt into another rock, with what perhaps looks like a metamorphic aureole in the intruded rock (yellowish band) but this could be a remnant of weathering. There appear to be chilled margins in the intrusion which look to have been weathered out preferentially, the weathered out area in the centre could indicate this intrusive pipe was used as a conduit more than once. The rock into which the intrusion occurred looks to be larger grained. It would be interesting to see the reverse side of this rock and fresh surfaces for a better stab at identification of rock types. Both dry and wetted images of the rock would also be handy in future.
It looks man-made to me. Specifically, it looks like the outer rock was (perhaps naturally) smoothed before it was (not naturally) cut and then the inner rock added.
Note the smooth outer rock. It looks water-worn, and its smoothness goes right up to that inner block (absent those cracks). But, at the same time, the inner block seems to protrude, protecting the nearby parts of the outer rock from abrasion (note the light-colored crust adjacent to the inner block). If that's true, then that part of the outer rock was smoothed before the inner block was added. I have no clue how this could have been done naturally.
I also can't believe that such a small intrusion could be done naturally without splitting the original rock.
That said, an oblique view of the apparent intrusion would be helpful.