It is a piece of flint that contains a fossil.
Flint is often formed around a fossil; the fossil acts as a nucleus.
But I am not sure what the fossil is, partly because it is difficult to be sure of its geometry in 3D from its appearance on the broken surface.
Maybe one of our other fossil-folks recognizes it, or can hazard a guess...
Oh - I should add: fossils can include not just plants and animals, but traces made by them. Some flints are in the form of burrows. I don't think the fossil here is a trace fossil; I mention it just for general knowledge.
BTW, the arc-shaped 'ripples' on the broken surface are nothing to do with fossils; they are an aspect of the conchoidal style of fracture that flints often display.