IMO, this is a tough one. If natural, in cases like this, people point to differential weathering to explain the depression or groove. In other words, a softer mineral weathered away quicker then the surrounding harder mineral/ rock, and left that groove where it weathered away. But, double clicking your photos to get max enlargement, it's very hard for me to think that is the case. It is hard for me to see that groove as anything other then a groove that was pecked and ground into that rock by somebody. It looks like a miniature axe or hammer. Yet, there is no wear at what would be the business end. No wear if it were a hammer, and no bit if it were an axe. That said, I'm still inclined to see that groove as man made. The entire piece seems water worn, so no real evidence of pecking survives.
All this said, maybe it is 100% natural. Since the side shot of the groove shows a "sharp" edge, rather then smoothed over, well then, maybe it's simply a cool looking rock mimicking the look of a full groove artifact of some type.
