Could the locket that Harry and Dumbledore found in Half-Blood Prince actually be a real Horcrux?

I don't think it was unless it's transfigured to look like an insignificant item. Good thought, but as we know voldemort wanted his horcruxes to be of importance slytherines locket, is a very important piece to the collection, I don't think voldemort ever even thought that anyone would ever realize that he had horcruxes in the first place.