Is this a tool?

[gentleone444]

Probably natural but it does look similar to the wear point I've seen on hammerstones.

 

[quito]

I don't know what it would be if it were.

 

[dognose]

i don't see any indication of use or alteration by native Americans from the photos.

 

[willjo]

Looks like some net weights a person finds on another forum, they have a notch like yours on opposite sides of the rock.

 

[Treasure_Hunter]

Sorry, nothing there says worked by man.

 

[MAMucker]

Nope