Did I find a fossil?

I can't really tell from the picture. It could be, but a lot would depend on the type of rock. Also, the color is so different from the surrounding rock, I might think it would be a crystal or embedded mineral of some type. Not sure, but I have no professional backing so this is just a guess. I think, however, that it would be pretty unlikely that you would split a rock at random and recover a leaf imprint... those are pretty thin and not that often found in common sedimentary rocks.

I can see the crystalline texture. It doesn't look like a fossil. The brown coloration looks like weathering from fluid flow along a crack. The gray looks like a fresh surface of the actual rock before fluid flow had reached it to cause weathering and discoloration. Along the edges of the gray rock I think I see individual mineral grains, supporting the idea that water was leaching in along boundaries between minerals.

A leaf fossil will usually be dark gray due to the carbon in the leaf.

I can't say for certain, but I'm most sure that that is a xenolith. The parent rock has been intruded by an igneous intrusion, what you see it is molten rock that invaded the large brown rock before cooling. If that was a fossil by the way, you would most likely see striations or a larger grain within the host rock. Since I study fossils and that in no way represents any I know of, I would most certainly say that that is not a fossil.

Ah... is the natural color of the rock that dirty brown, or the slate grey?

Looks to me like the rock had slowly been fracturing along that plane for a long time, and dirt had gotten into the crack to discolor the rock. The grey area is the fresh fracture that happened when you kicked it.

In other words, probably not.

I saw the picture and I can't tell. Is there any veining or detail on the leaf shape? I've never found one that was silvery like that, but who knows! Hope you get a definitive answer. Mine is, "probably not".