How big is the stone? It's hard to tell how far apart the grooves are without knowing how big the stone is.

As they are very evenly spaced they could actually be natural - if it's volcanic rock it could be from a crystalline cooling type effect. This doesn't devalue the stone. This could be why the stone was picked and used in or near a stone circle.

Only the nearest line in the picture looks as if it ends short of the edge of the stone - the rest go under the grass, so it's hard to tell.