Many people just post nonsense pieces, but yours is a little intriguing... With that said, I'm leaning towards not intentionally man made, but I can certainly be wrong. My logic is because the lines are so thick but I don't see evidence of multiple score lines (at least in the photo.) I can't imagine the sharpened edge of a knife, scraper, whatever could make such a mark so even without seeing multiple score lines from sawing back and forth. Also, there's another line following the curve of your finger... I can't imagine if someone was carving something into a rock, they would put a line right on the cusp of the edge. The lines are intriguing but I say not intentional.