About 3"X2"X1.25"
Found in a friend's yard, a friend who collects fossils.
He doesn't remember where he got it.
But it is most likely from the Miocene Greeensands of the Gay Head Cliffs, and represents some bone of a marine mammal.
The end is extremely smooth and shiny, looks a lot like a bone. It is a heavy chunk of rock.
A seal bone? Small whale?
This 1934 study is the best reference that I've found for the 100-million year old geology of the Gay Head Cliffs where bones of a camel and a horse were found in a glacial era deposit, and many marine fossils have been found in the greensands.
https://archive.org/stream/mobot31753003559371/mobot31753003559371_djvu.txt