From a composition point-of-view, the pebbles do not fit in with this image. They have no tie-in. The chipped blades are made of obsidian and chalcedony or light flint, I think. The pebbles have no relation to those materials. I think more effeort could be put into the composition. Then add to that all that has been said about lighting etc.
I concur with what the Doc says. I'd add to it that the use of a polarizer might help with the rendering of the oblique surfaces on the black arrowhead. But my major reservations about the image lie in the very static composition and the lack of separation of the black arrowhead from the pebbles. I think the 5.5 is a decent score regardless.
I think you did fairly well here with a 5.5. A few thoughts:
1) The black arrowhead blends too much with the black rocks of the background. It also has that funny reflection down the middle.
2) The composition is very centered. That doesn't give us any dynamism in the shot which is already static being basically a still life of small objects.
3) The white arrowhead is almost blown out.
Keep working at it. 5.5 isn't a bad score at all on this site. I see you've done better, but we can't win em all, eh?